I have a 2006 WRX with 2.5 engine and have a decat downpipe and only the centre cat still in place. I have sailed through the last 2 MOT emissions tests. Exhaust is a Scoobyworld decat downpipe, Scoobyworld centre section and Scoobyworld Ninja2 backbox. With a TurboTechnics T36 Hybrid turbo, a 2008 STI intercooler, Walbro 255 l/h fuel pump and a K&N Apollo cold air system. Enginetuner did the Racerom remap and got around 360bhp and 390 ft/lb torque out of it all. There is performance to be had, it costs a bit but a good tuner will keep it all safe and good for you. Any performance increase has risks so be prepared!! Keep it serviced and it will keep you smiling for years.