i am fitting a cat replacement pipe on my import 96 wrx (back cat) does anyone know will this pass or fail mot . and what sort of bhp willthis gain, its 260bhp standard and has induction kit and magnex back box. cheers if anyone can throw any figures my way

Strictly speaking it shoudl fail MOT, but that doesnt always happen. Saying that, God knows what will happen in these new MOT centres. Some cars may not make it out alive.
As for power, I wouldnt even guess on HP figures the std cars actually have, but you should feel the improvements with the downpipe.. It is a good mod, and very worth doing.