Most believe that a 2-1/4 set up is the best for an all motor set up and 2.5 is for turbo only. The counter agruements I have heard from tuning shops is that 2.5 gives you good flow and can't hurt you even when starting with basic bolt ons and 3 inch should be used for any turbo application.
I don't believe the diameter of your exhaust tubing will make or break your emissions testing. With more space the sound should resonate more and be deeper given the same muffler application. Just in theory I guess.
I think you can have some argument as to what is best for the focus but I don't know if anyone has dyno'd the same setup with 2.25 and 2.50 back to back and noted any gains either way. I'm happy with my 2.5 and all I have is bolt ons.