Exhaust & Catalyst Advice please

[craigdmcd]

My mate had some bad luck last week and No3 big end bearing went on his MY2000 classic turbo. The damage isn't bad, but just enough to require engine work. Rather than splashing the cash for this, he has opted to go and buy a nearly new engine out of a 54 plate WRX. All of the change is straight forward ( a few cam and crank sensors etc). But now to the question posed by this thread. The new engine seems to have a small (Pre?) catalyst for an up-pipe before the turbo, which I am currently going to change for the existing up-pipe. We are talking about uprating the car and de-catting it, will putting this small catalyst back in help the emissions much and does it affect turbo spool up speed? Also, I'm looking for advice on what exhaust system to fit, obviously it has to increase performance, but am not looking for something that would make a WRC Impreza S11 sound quiet. The sound of my wife's WRX PPP is very appealing. I have heard good things about H&S systems. All thoughts and advice will be gratefully appreciated.

Craig