o my standardish '93 wrx what is the best diameter exhaust (for back pressure etc)?

I have a 3inch downpipe (hks) but are thinking this is probably to big for approx 300bhp!

Would exhaust flow be ok if I fit a 2.5 inch centre section to this downpipe?

cheers guys
Mark.

 

300 bhp
keep the 3" down-pipe and stick with the 2.5 centre and back box , , only 340+ and you'll need a 3" system