There isn't much chance you will realize your engines power with 2.5" exhaust.It just can't handle the flow.
I have a '67 rs/ss camaro with a 454 with 525-550 hp,4 speed.
It had a fairly new 2.5" exhaust and it felt retricted from about 4.5k rpm and up. I replaced it with a 3" mandrel bent exhaust with 3" core dynomax mufflers.
NO COMPARISON
The restiction changed to wheelspin from 4.5k up.
I don't know what the hp increase was but it felt like a whole different engine at every rpm.
Try it,you'll like it.