Aftermarket (custom) exhaust. Questions

[GT3]

..not to mention that with a 3" DP, you'll lose all your torque (not gain).

[Grant]

does anyone know how would a 2.5" downpipe with a 3" catback perform?

[Bubu]

Its probably better to go the other way around 3' dump and 2.5' cat back. As the exhaust gases flow through the length of the exhaust system it cools down.
Heat expands and cold contracts, so the hottest part of the exhaust, being right after the turbo, is where you would want the larger diameter pipe.