top swap- the action of removing your whole top end of a N.A 3.8 and swapping that top end from a supercharged 3.8.

if your n.a, i say 2.5" with 2.5" dp, if your s.c'd I would get a 3" dp if you do 3" exhaust. There won't be any performance gains from the catback. You will get some from the u-bend delete in the aftermarket dp's.