I will have a dyno tune very soon.

For my car(B16A4) now has a 4-1 S-S header + 92 civic si original cat + 2.25' cat back system + xforce twinloop muffler.

do i better need change to a 2.25' high flow cat to gain some more power from dynotune? big difference?
If i dont change the cat, how bad will it force?