2.5" exhaust vs 2.25 or even 2"

I’d bet good money on, yes, there will be a gain.
If it were I that is doing what your doing, even if the engine was otherwise dead stock, I’d go to a 2-1/2 system with an X or H pipe.

Dyna Max Super Turbo mufflers would be used. The short version for some muscle sound, the long ones @ 20 inches in length (if you can fit them) for quieter sound.