Best 2015-2021 WRX STI Cat-Back Exhaust: HKS vs Borla

A WRX STI cat-back exhaust is the single most popular upgrade for the 2015–2021 VA-chassis STI, and for good reason: it wakes up the EJ257 2.5L turbo boxer's signature rumble, trims weight, and improves exhaust flow without touching a tune. Subaru rated the US-spec VA STI at 305 horsepower (310 for 2019–2021 cars) and 290 lb-ft of torque from the factory, and the restrictive stock cat-back is one of the first things enthusiasts swap. At NLP Performance in Tampa, FL, the STI exhaust question comes up constantly, so we lined up five cat-backs we actually stock — from a $1,413 MagnaFlow to a $6,300 titanium HKS — and compared construction, sound, tip configuration, and price so you can pick the right one the first time.

What is the best cat-back exhaust for a 2015–2021 WRX STI?

The best cat-back exhaust for a 2015–2021 WRX STI is the HKS Super Turbo Muffler Ti if budget is no object — it is full titanium and built to JDM race standards — while the Borla S-Type is the best value for most owners at $1,949.99. All five systems below are true cat-backs (they replace everything from the catalytic converter rearward) and bolt onto the EJ257 without cutting or welding. Below is a head-to-head of the five we stock, ranked by build and price.

How much horsepower does a cat-back exhaust add to a WRX STI?

A cat-back exhaust alone adds only a small, variable amount of power on a 2015–2021 WRX STI — realistically anywhere from negligible to about 5 wheel horsepower — because on the STI the main exhaust restriction lives upstream at the downpipe and up-pipe, not in the cat-back. The stock STI makes 305 hp and 290 lb-ft (310 hp on 2019–2021 cars), and because a cat-back only replaces piping from the catalytic converter back, it frees up some flow and trims weight without altering the tune. The big power numbers you see online — 30 or more wheel horsepower — require a downpipe (a turbo-back setup) plus an ECU flash. Treat a cat-back as a sound, weight, and throttle-response upgrade first, and a modest power upgrade second. The larger-diameter mandrel-bent piping in these systems flows better than the pinched factory routing, which is where the real-world seat-of-the-pants improvement comes from.

HKS Super Turbo Muffler Ti — The Titanium Halo Pick

The HKS Super Turbo Muffler Ti is a full-titanium, JDM-developed cat-back and the priciest STI exhaust we stock at $6,300.00. Titanium is why: HKS lists the system at roughly 29.5 lb — up to about 48% lighter than a comparable stainless muffler — and it wears the burnished, blue quad-tip look that Subaru and JDM fans instantly recognize. It is JASMA-certified with an HKS-published sound level around 85 dB, and it fits the 2015–2021 VAB WRX STI chassis. This is the halo purchase — a now-discontinued, hard-to-source titanium exhaust you buy when weight savings, exclusivity, and JDM heritage matter as much as raw dollars-per-decibel. Two caveats: HKS backs it with a shorter one-year warranty (versus the lifetime-class coverage on the Borla and MagnaFlow), and HKS lists the system as requiring a CARB acknowledgement, so confirm your local emissions rules before installing.

Borla S-Type — The Best Value Cat-Back

The Borla S-Type is our value pick at $1,949.99, and it is the cat-back we recommend to most STI owners. It is built from premium T-304 stainless steel — genuinely superior to the T-409 "stainless" many budget systems use — with ultra-smooth mandrel bends and Borla's multi-core muffler technology to maximize flow. The S-Type muffler is Borla's aggressive tune, and paired with 2.5-inch black chrome quad tips it looks and sounds the part on the VA STI. Best of all, Borla backs every system with a Million-Mile Warranty, which is effectively lifetime coverage. For the buyer who wants a proven, loud, corrosion-proof cat-back without a five-figure receipt, this is the sweet spot.

HKS LEGAMAX Premium — The Refined Daily Driver

The HKS LEGAMAX Premium is the exhaust for the owner who wants HKS quality with a more livable, refined character. Built from SUS304 stainless steel with dual 90 mm polished tips, LEGAMAX Premium is HKS's street-oriented line — deeper and more purposeful than stock, but tuned to keep highway drone in check so it stays comfortable on a daily-driven STI. It is JASMA-approved with an HKS-published sound level around 83 dB, making it the quietest of the three HKS systems here. At $1,990.00 it sits right alongside the Borla on price, so the choice between them comes down to sound preference and tip style: LEGAMAX Premium leans refined and understated, the Borla S-Type leans loud and aggressive. If you commute in your STI and value a mature, drone-controlled note, the LEGAMAX Premium is the pick.

MagnaFlow Competition — The Budget Champion

The MagnaFlow Competition Series 19361 is the budget champion of this group at $1,413.00, and it does not feel like a compromise. It is built from 409 stainless steel with fast-flowing 3-inch mandrel-bent main piping, straight-through mufflers, and a quad split rear exit with 4-inch polished, double-wall, angle-cut tips, and it is backed by MagnaFlow's Limited Lifetime Warranty. MagnaFlow describes the character as aggressive on the outside and moderate in the cabin, which is exactly what a lot of STI drivers want — a serious growl when you are on it, but a tolerable interior on the highway. It fits 2015–2017 WRX STI 2.5L models. If you want the biggest visual and audible upgrade per dollar, start here.

HKS Hi-Power Muffler — The Classic Single-Exit Tone

The HKS Hi-Power Muffler is the classic HKS street cat-back, and it delivers the loud, full single-exit tone that put HKS on the map. At $1,900.00 it undercuts the LEGAMAX Premium slightly and takes a different design approach: a single large-diameter (roughly 97 mm) burnt tip rather than dual or quad tips, and HKS lists it around 33 lb versus about 46 lb for the stock muffler. It is the loudest of the three HKS systems here at an HKS-published level near 90 dB, so it is the pick for owners who want maximum presence and the traditional single-tip look — just expect more highway drone than the LEGAMAX Premium. HKS builds it from 304 stainless steel.

Cat-back vs axle-back vs turbo-back: what is the difference?

A cat-back exhaust replaces every pipe and muffler from the catalytic converter rearward, while an axle-back only replaces the last section behind the rear axle and a turbo-back replaces everything from the turbocharger back, including the downpipe. On a 2015–2021 WRX STI, a cat-back is the sweet spot: it delivers a full sound and visual change plus a modest flow gain without the emissions and tuning complications of a downpipe. Axle-backs are cheaper but change less of the sound and flow; turbo-backs unlock the biggest power (30+ whp with a tune) but almost always require an ECU flash and delete the factory catalyst, which has emissions consequences. Every system in this guide is a true cat-back, so you get the sound and flow upgrade while keeping the factory catalytic converter and your stock tune.

What does a WRX STI cat-back sound like?

A WRX STI cat-back amplifies the EJ257's boxer character — more volume under acceleration and a raspy, rally-bred edge on overrun — but it helps to know where the classic Subaru "rumble" actually comes from. That uneven burble is produced by an unequal-length exhaust header, and the 2015–2021 STI ships with an equal-length header from the factory, so a cat-back changes the exhaust's volume and tone without adding the old-school warble (owners who want that sound add a UEL header upstream). The main thing to weigh is highway drone: aggressive systems like the Borla S-Type and MagnaFlow Competition trade some cabin quiet for volume, while the HKS LEGAMAX Premium is engineered to keep drone in check for daily driving. If you want maximum theater, go loud; if you commute long distances, choose a system tuned for a calmer cruise. At our Tampa shop, we always ask customers how many highway miles they drive before recommending one.

Installation and fitment notes

Every cat-back in this guide is a bolt-on install that fits the factory hangers and requires no cutting or welding, and most STI owners can complete it in one to two hours with basic hand tools and a set of jack stands. Because a cat-back does not touch the tune, there is no ECU flash required and no check-engine light on a healthy car. A few fitment notes matter: the Borla S-Type lists 2015–2019 fitment, the MagnaFlow Competition covers 2015–2017, and the HKS Super Turbo Muffler Ti is listed for 2015–2021 VAB WRX STI, so always confirm your exact year before ordering. We recommend replacing the exhaust gaskets during the swap and applying anti-seize to the hardware — Florida humidity is hard on fasteners, and it makes the next job easier.