Remove the cat and re-program your ECU to run without it. change the exhaust not just backbox.this is a easy and cheapish way to get good bhp gains, a front mounted intercooler is a must for subaru impreazas, this will lower you charge air mass and temp and at the same time increase your boost pressure, there is a small downside to the front mounted intercooler and that is a increase in lag but nothing to bad. fit a boost controller really great gadgets!! at this point a uprated fuel pump and rail would be a good investment.
If you what to increase the horse power above what this will give you i would recommend improving the engine internals this is not cheap mind!! the ultimate would be a stroker kit but that is only my opinion.
Apex Stainless Brake Lines Hoses SUBARU IMPREZA WRX 01 02 03 04 05, STAINLESS SS TURBO DOWNPIPE DOWN PIPE 02-07 SUBARU IMPREZA WRX/STI TURBOCHARGED, 3" BELL MOUTH DOWNPIPE SUBARU IMPREZA WRX/STI STAINLESS STEEL DOWN PIPE EXHAUST , SUBARU IMPREZA 2.5RS 2.5L STAINLESS STEEL PERFORMANCE HEADER/EXHAUST/MANIFOLD, 02-07 SUBARU IMPREZA WRX/STI EJ25 TURBO 1PC STAINLESS STEEL EXHAUST HEADER 2.5L, '02-07 SUBARU IMPREZA WRX STI STAINLESS STEEL TURBOBACK EXHAUST & UP PIPE , *NEW Subaru Impreza STI Spec C STAINLESS STEEL WATCH*, NEW Subaru Impreza WRX STI Steering Wheel CUSTOMIZE STAINLESS STEEL MENS WATCH, SUBARU IMPREZA WRX/STI GDB EJ20/EJ25 2 PCS STAINLESS STEEL EXHAUST RACE HEADER, STAINLESS MANIFOLD HEADER UP PIPE/EXHAUST JDM 02-07 SUBARU IMPREZA WRX/STI EJ25, SUBARU IMPREZA WRX GC/GD 2.5RS 2.5L RS NT EJ25 STAINLESS EXHAUST HEADER+GASKET, 02-06 SUBARU IMPREZA WRX/STI GDB TURBO STAINLESS EXHAUST CHROME HEADER+GASKET, 2-PC STAINLESS STEEL MANIFOLD HEADER UPPIPE/EXHAUST 02-07 SUBARU IMPREZA WRX/STI, Subaru Impreza Polished Stainless Steel Coil Pack Cover OR Can be Powder Coated!, STAINLESS SS DOWNPIPE DOWN PIPE 08-11 SUBARU IMPREZA WRX/STI 2.5L TURBOCHARGED, STAINLESS SS RACING MANIFOLD HEADER/EXHAUST 97-05 SUBARU IMPREZA 2.5L RS EJ25 NA, 1997-2005 SUBARU LEGACY/IMPREZA WRX RS 2.5L STAINLESS STEEL HEADER EXHAUST 98, STAINLESS STEEL EXHAUST HEADER/MANIFOLD 02-07 SUBARU IMPREZA WRX/STI 1PC EJ25, Stainless Steel Brake Lines Hoses SUBARU IMPREZA WRX STI 04 05 06 07, Stainless Steel Brake Lines SUBARU IMPREZA WAGON Outback Forrester 93 94 95 96, Stainless Steel Brake Lines Hoses Hose SUBARU IMPREZA RS TS 03 04 05, Stainless Steel Brake Lines SUBARU IMPREZA RS ERX 01 02 03 04 05, Stainless Steel Brake Lines Hose Hoses SUBARU IMPREZA WRX 01 02 03 04 05, Stainless Steel CLUTCH LINE SUBARU IMPREZA RS WRX 98-04, Subaru Impreza WRX / STI 02-07 - 3" pipe T304 MEGAN Stainless steel Downpipe, Intercooler Stainless Steel Piping T201 45° 3.5"16Gauge Nissan Subaru Impreza , Header Stainless Steel Pipe45°2.5" 2 Couplers 4T-Bolt Subaru Impreza Fit S2000, Stainless Steel Exhaust Pipe Piping 3"X7" 2.9"ID 3"OD Subaru Impreza Toyota Neon, OEM 02-05 Subaru Impreza WRX STi Stainless Brake Hoses, 02-07 SUBARU IMPREZA WRX STI EJ25 TURBO STAINLESS STEEL EXHAUST HEADER+UP PIPE,
