Best 2015-2021 WRX STI Cat-Back Exhaust: 6 Compared

Choosing a WRX STI cat-back exhaust for the 2015-2021 VA chassis comes down to three things: how much titanium you can stomach paying for, how loud you want it, and whether you can live with highway drone. The factory EJ257 2.5-liter turbo boxer makes 305 horsepower (310 hp on 2019-2021 cars) and 290 lb-ft of torque, but Subaru bolts it to a restrictive, emissions-tuned muffler with roughly 2.25-inch piping. A cat-back is the single easiest bolt-on to fix the sound, drop weight and open up the exit — and it is the first mod most STI owners make. At NLP Performance in Tampa, FL, these six systems are the ones our Subaru customers cross-shop the most.

The Best 2015-2021 WRX STI Cat-Back Exhausts at a Glance

Every system below is a direct bolt-on cat-back for the 2015-2021 Subaru WRX STI (VA chassis) — no cutting or welding. They step the exit up from the restrictive OEM setup to a full 3.0-inch path (the HKS Ti uses its own tuned diameter). The difference between them is material, tip design, and how each brand tames the boxer engine’s notorious highway drone. Prices reflect live NLP Performance listings at time of writing.

HKS Super Turbo Muffler Ti: The Titanium Benchmark

The HKS Super Turbo Muffler Ti is a full titanium cat-back that weighs roughly 13 kg — about half the 26 kg of the equivalent stainless HKS system and a major drop from the ~23 kg OEM muffler. That is the entire pitch of a titanium exhaust: motorsport-grade weight savings at the rear of the car, plus the prized burnt-titanium tip coloring that heat-cycles from gold to purple to blue. HKS rates it at roughly 85 dB with a low-back-pressure design that keeps it civil at cruise, then opens into a raspy, deep race note under throttle. HKS catalogs this unit under the JDM VAB / EJ20 application, but it is the same physical cat-back that bolts to the US-market 2015-2021 WRX STI (EJ257).

Borla S-Type: The Aggressive Stainless Benchmark

The Borla S-Type is the loud, drone-free stainless pick and Borla’s most aggressive street tune. Built from T-304 austenitic stainless steel and backed by Borla’s Million-Mile warranty, it runs a 3.0-inch inlet splitting to dual piping and exits through quad 3.5-inch black-chrome tips. Borla’s “S-Type” sound rating is engineered to bring out the full boxer rumble while staying “aggressive without drone” at highway speed — the sweet spot for owners who want their STI heard without a resonant cabin boom on long drives. At $1,949.99 it is the priciest stainless system in this group, and the tone justifies it.

CORSA Sport: Best for Drone-Free Daily Driving

The CORSA Sport is the pick for owners who want aggression on throttle and total quiet on the highway. Its headline feature is patented RSC (Reflective Sound Cancellation) technology, which is engineered to eliminate cabin drone at cruise while still delivering a loud, throaty note under load. Built from 304 stainless with 3.0-inch piping and twin 3.5-inch black PVD Pro-Series tips, CORSA cites roughly a 68% flow increase over stock at the Sport tune (its louder of two sound levels). It is backed by a lifetime warranty. At $1,820.69 it lands just under the Borla while offering the most engineering focus on drone elimination.

Invidia Gemini R400: The Enthusiast Middle Ground

The Invidia Gemini R400 is the balanced, resonated pick that slots between mild and loud. Made from SUS304 stainless with 1.5mm mandrel-bent 3.0-inch piping, CNC flanges and a resonated mid-pipe, the R400 sound tune sits between Invidia’s tamer Q300 and its raucous N1 — an aggressive note that avoids harsh highway cabin noise. It exits through quad 4-inch black slash-cut tips and bolts to the OEM downpipe flange with included gaskets and hardware. At $1,735.43 it is one of the best all-rounders here for owners who want presence without committing to the loudest option.

AWE Touring Edition: Rumble With Zero Drone

The AWE Tuning Touring Edition is the engineered no-drone choice with a proven power claim. Its 180 Technology uses a dedicated resonator to cancel drone, delivering “rumble without rasp or in-cabin drone” — AWE’s milder of two exhaust lines (the Track Edition is the loud one). It is built from T304L stainless with 3-inch, 0.065-inch-wall CNC mandrel-bent tubing and 102mm chrome-silver tips. AWE advertises gains of up to +11 hp and +11 lb-ft, though independent cat-back-only testing on the EJ257 typically shows smaller peak numbers (more on that below). At $1,595.00 it is the value leader among the drone-cancelled premium systems.

MBRP Armor Pro: The Budget Single-Exit Pick

The MBRP Armor Pro is the budget-friendly, single-exit outlier and the value champion of this comparison. It is a 3-inch T304 stainless cat-back with a single rear exit and a large burnt-end tip — a distinctly different look from the quad-tip crowd, and MBRP claims roughly a 34% flow increase over stock. Its street tune is aggressive on throttle but not obnoxious at cruise, and it carries a lifetime warranty. At $734.99 it is less than half the price of the next-cheapest system here, making it the obvious choice for owners who want a real T304 upgrade on a tighter budget.

Does a Cat-Back Exhaust Add Horsepower on a WRX STI?

A cat-back exhaust alone adds only a few horsepower on the EJ257 — it is primarily a sound-and-weight modification, not a power modification. Independent testing tells the honest story: a Mishimoto dyno on a stock-tune WRX measured just +2.93 peak wheel horsepower but a more useful +9.65 lb-ft of torque from a cat-back swap. Manufacturer claims like AWE’s +11 hp / +11 lb-ft are best-case figures. The real restrictions on a turbo Subaru sit upstream — the turbo, the OEM catalytic converter, and the factory downpipe — not in the least-restrictive cat-back section.

If horsepower is the goal, the money goes to a catted or catless downpipe plus an Accessport tune first. A downpipe and Stage 2 tune commonly put a US STI around 250 wheel horsepower on pump gas, and the EJ257 short block is generally considered safe to roughly 400-450 whp on stock internals. Think of the cat-back as the finishing touch on that build: it sets the sound and sheds weight, while the downpipe and tune make the power. A cat-back with no tune yields negligible peak gains.

Sound and Drone: What Actually Makes a Subaru Rumble

The signature Subaru boxer rumble comes from the factory unequal-length (UEL) headers, not the exhaust or the flat engine layout itself. Uneven runner lengths make exhaust pulses arrive out of sync, producing that warbly burble Subarus are famous for. Switching to equal-length headers smooths the pulses and makes an STI sound generic, which is why many owners keep the UEL manifold. A cat-back changes volume and tone, but the header sets the fundamental character.

Drone — the resonant highway boom around 2,000-3,000 rpm — is the number-one complaint on straight-through Subaru systems. In this group, the engineered drone-free picks are the CORSA Sport (RSC), the AWE Touring Edition (180 Technology), the Borla S-Type (aggressive without drone), and HKS’s quieter Hi-Power SPEC-L II. The Invidia R400 uses a resonated mid-pipe to stay civil, while the HKS Super Turbo Ti is deliberately quiet at low rpm but opens into a loud race note up top. On a daily-driven UEL STI, the RSC and 180 systems are the safest bets against cabin boom.

How Hard Is a WRX STI Cat-Back Install?

Installing a cat-back on a VA-chassis WRX STI is a DIY-friendly bolt-on that takes about 1-2 hours — one of the easiest mods on the platform, with no cutting or welding on any system here. You will need a jack and jack stands (no lift required) and a basic socket set: 10mm for the small clamps, 13mm for the 3-inch band clamps, and 12mm/14mm for the spring bolts. A breaker bar helps with stubborn hardware.

Two tips save headaches. Spray penetrating oil on the flange bolts and rubber hangers the night before — rust-belt cars often have seized OEM hardware. Drop the rubber hangers first to free the muffler, and always install a fresh downpipe-to-cat-back gasket to avoid leaks. Most kits, including the Invidia R400, ship with new gaskets and hardware.