My Buddys '01 (bug eye) Jap WRX is giving bother - Mods are 1 cat removed and straight through exhaust system. Blitz dump valve and turbo timer.

The problem he had was when he bought it from a subrau main dealer in belfast that the engine light was on in it the whole time and they told him it was the rear oxygen sensor and they would post him one down - the car drove perfectly and a month later they sent him the sensor and the day before he got it fitted it started to give bother - it was down about 25% power and didnt seem to be boosting properly. He got the sensor fitted and no good - it's still the same and the local subrau garage put it on the computer and can find no faults-
Any idea's?

Thanks
James