for turbo charged cars, the less resistance in the exhaust system, the more power youll get. therefore, no exhaust is the best, a.k.a. open downpipe. but in a world without laws, that would be everyones setup.
also, if you are running low boost/ mild power increase (under 7 lbs/ under 250hp ) then a 2.5" system will be fine.
as for a test pipe or cat. it depends on if you want to be legal or not. with a high flow cat, you get pretty good flow, while still retaining emissions equipment. but a test pipe WILL flow better. probably by about 10% though, so its up to you if you care about the extra 20 or so CFM extra you can push through your exhaust. Personally, i dont think test pipes are worth it, unless its a race only car that doesnt need to meet emissions laws.
if you are wanting tons of power (300+) then i would suggest at least a 3" exhaust. and it MUST be Mandrel Bent. most aftermarket systems, if not all of them, are Mandrel bent. if you go to a muffler shop, and want something like a 3.5" or 4", then make sure they use mandrel bends.
a 4" setup would be for something in the 400+ range.
make sure your entire exhaust system, from the turbo to the tip, is all the same diameter, even the cat. this will get rid of any bottlenecks that can rob power from your boosted car.
