you want as little back pressure as possible. whoever says loosing back pressure kills torque is wrong, you dont want backpressure. the problem is, people put bigger pipe to reduce back pressure, which kills VELOCITY. its when you reduce your exhaust velocity (big pipe) that you loose torque. scavenging has nothing to do with the cat back, ONLY has to do with headers/exhaust manifolds. the exaust comming out of one header tube can actually help to create a vacuum to "suck" the exhaust out of the next tube, adding velocity (torque) which is why its very difficult to make GOOD headers, theres a ton of engineering that goes into headers to do this. this is why a ported exhaust manifold will never be as good as a properly designed header, they can come close to cheap headers
and a big air filter is as bad as big pipe on your exhaust, for the same reason, velocity. if you think of it as a vacumm, put a 4" extension on the end of your shop vac, and try to suck stuff up, its not nearly as effective as the same vacuum with a 3", you have to find the balance
2.5" is overkill for a 93 se that makes about 170hp. you'd be better off with 2.25" and a strait through muffler. mufflers like flowmaster sound pretty good, but they are not a performance muffler, they have baffles which deystroy airflow/velocity, but are "tuned" for a good sound.
modded & dropped 95 Aurora - SOLD for a 2001 Alero GLS - which is now being sold for a 93 SSEI WHICH got sold for a modded 98 regal GS!!!
98 regal GS:
3.4 / thrasher intake / SLP powerflodual exhaust / 3-2.5" downpipe w/ highflow cat / DHP 1.0 / afc 2.2 / gmpp handling package / other small things