Best Exhaust for 2015-2021 Subaru WRX STI: 5 Compared

The best cat-back exhaust for the 2015-2021 Subaru WRX STI does three things at once: it frees up the EJ257 2.5L turbo boxer, drops weight behind the rear axle, and gives the car the rally-bred rumble owners actually buy an STI for. The VA-chassis STI leaves the factory with 305 hp and 290 lb-ft of torque (310 hp from 2019 on), and a restrictive stock exhaust that muffles most of that character. Swap the cat-back and you unlock flow, tone, and—with a titanium system—real weight savings. At NLP Performance in Tampa, FL, the five systems below are the cat-backs we return to again and again for the VAB STI. Here is how the HKS Super Turbo Ti, HKS LegaMax Premium, Borla S-Type, HKS Hi-Power SPEC-L2, and MagnaFlow Competition Series stack up on material, sound, weight, and price.

Which Cat-Back Exhaust Is Best for the 2015-2021 WRX STI?

The HKS Super Turbo Muffler Ti is the best overall cat-back for the 2015-2021 WRX STI, but the right pick depends on your priorities. Choose titanium (HKS Super Turbo Ti) for the lightest weight and the most exotic tone, a refined silent-chamber design (HKS LegaMax Premium) if you commute daily, American-made T-304 stainless with a lifetime-class warranty (Borla S-Type), a loud JASMA-legal JDM note (HKS Hi-Power SPEC-L2), or maximum value (MagnaFlow Competition Series). Every system here is a direct bolt-on cat-back that fits the 2.5L EJ257 STI without cutting. The table below compares all five head-to-head.

HKS Super Turbo Muffler Ti: The Titanium Benchmark

The HKS Super Turbo Muffler Ti is the lightest and most exotic cat-back you can bolt to a VAB WRX STI. It is built entirely from TP270 titanium, weighs just 13 kg (28.7 lb)—roughly 57% of the stock system and up to 48% lighter than the stainless Super Turbo—and steps the main pipe up from 74.7mm before splitting into quad 90mm "W-Tail" tips. HKS tuned it for a quiet, comfortable cruise around 85 dB at idle that turns into a raspy, unmistakably JDM snarl the moment the turbo spools. It is the flagship of this test at $6,300.00, and because HKS has discontinued it, remaining stock is genuinely limited.

HKS LegaMax Premium: Refined Power Without the Drone

The HKS LegaMax Premium is the best choice for the STI owner who wants more flow and tone without waking the neighborhood. It is built from polished SUS304 stainless with a 75mm midpipe that splits into dual 69mm piping before exiting through quad 90mm HKS-engraved tips. HKS packs in a 101.6mm silencer and a dedicated silent chamber, holding sound to a measured 83 dB while the ~35 lb system still opens up the EJ257 on boost. At $1,990.00 it is the drone-free daily-driver pick, and its OEM-location welded hangers make it a true bolt-on.

Borla S-Type: American-Made Aggression

The Borla S-Type is the best-balanced stainless cat-back for the WRX STI and our value-conscious pick for most builds. Handmade in the USA from premium T-304 stainless, it runs a 3-inch inlet into dual 2.25-inch piping and exits through blacked-out 2.5-inch black chrome quad tips. Borla's patented straight-through and multi-core technology brings out the boxer rumble with an aggressive tone that stays drone-free on the highway—owners describe it as mean on cold starts and quiet while cruising. It carries Borla's Million-Mile Warranty and lists for $1,949.99.

HKS Hi-Power SPEC-L2: JASMA-Legal JDM Loudness

The HKS Hi-Power SPEC-L2 is the loudest street-legal system in this comparison and the pick for drivers who want maximum presence. It is built from SUS304 stainless with a 65mm main pipe splitting to dual 50.8mm, an Advantex wool-packed silencer, and a single 94mm tip. The result is an aggressive 89 dB note (a civil 67 dB at idle) that still holds JASMA street-legal certification in Japan. At just 10.4 kg (about 23 lb) it is the lightest stainless system here—54% lighter than stock—and it lists for $1,800.00.

MagnaFlow Competition Series: Best Value Cat-Back

The MagnaFlow Competition Series 19361 is the best-value cat-back for the 2015-2017 WRX STI, delivering a competition-grade note for $1,413.00—the lowest price in this test. It uses fast-flowing 3-inch mandrel-bent stainless piping, a single straight-through 14-inch muffler body, and a quad split-rear exit with 4-inch polished double-wall tips. MagnaFlow tunes it for an aggressive exterior tone with only moderate interior volume, and it backs the system with a limited lifetime warranty. Advanced 3D-scanned fitment makes it a true bolt-on for a driveway install.

How Much Horsepower Does a Cat-Back Add to a WRX STI?

A cat-back exhaust alone typically adds about 5-15 wheel horsepower to a 2015-2021 WRX STI, with the biggest gains coming from reduced backpressure and improved exhaust velocity higher in the rev range. The EJ257 stays in its restrictive stock tune until you pair the cat-back with a downpipe and a Cobb or EcuTek retune, at which point the exhaust upgrade contributes to a much larger 40-60+ wheel-horsepower package. On its own, the real payoff of a premium cat-back is throttle response, tone, and weight: the titanium HKS Super Turbo strips roughly 43% of the stock system's mass, and every kit here flows better than the factory muffler. Think of the cat-back as the foundation of a Stage 1 or Stage 2 STI build rather than a standalone power adder.

Fitment is straightforward across the VA generation. All five systems bolt to the 2.5L EJ257 STI, and the HKS Super Turbo, LegaMax, and Hi-Power SPEC-L2 also cover the JDM VAG WRX S4 and VAB STI. Note that CARB acknowledgement is required on the HKS systems, so California owners should confirm local emissions rules before installing. For everyone else, plan on a 60-90 minute driveway job with basic hand tools—these are true bolt-on cat-backs that reuse the OEM hanger locations.