"I'd think a true 2.5" exhaust would only net substantial gains on a highly modified motor"

Whaaaa? A true 2.5" exhaust like Hytech nets usually 190 whp with an extremely broad torque band....

Find me a dyno of someone making 190 whp on the stock exhaust with only intake header and cat....you'll be looking for a while...

2.5" is the way to go.