The resonator has a 1.875" baffled core and it's on 2.25" pipe.
You could say it's quite a flow restriction!
It is the second most restrictive piece of the exhaust after the POS crush flange in the Y-pipe.
Resonator replacement (straight through NON-louvered or split core) is worth 2-3 FWHP/TQ throughout the power band.
Optimized Y-pipe is worth 5-8 FWHP/TQ throughout the power band. (gains with a chip {i.e. timing!} have seen double digit TQ)
The cat is actually 2.5" in/out (on 2.25" pipe) with a very decent frontal area on the honeycomb. It's not much of a flow restriction.
High flow cat might be worth 1 maybe 2 FWHP in the upper rpm range. Provided of course you have an optimized Y-pipe.
Your thinking about "needing" back pressure is incorrect.
An ideal exhaust would have minimal to no back pressure and the highest exhaust velocity while sustaining enough volume to achieve maximum power.
You DO NOT want back pressure.
It's part of the trade off to achieve velocity (piping size) and volume control (mufflers)
