2.25" - 2.5" is usually ideal for a well modded car. 3" on a 4-banger (and some 6&8 cylinders also) is just overkill period. there will be no smooth flow at all which will actually work against you.

 

as someone stated earlier, no cat = illegal. At least that's the law here in california. Plus the sound of your exhaust system will dramatically increase. I only straight-piped the resonator, but left the cat on...that increased the sound by something like 2x-3x what it was originally. So no cat is gonna be REALLY loud

 

bah. on boost more than 15psi 3" isnt too much. it just wont be as good as a 2.5.

 

if this is ur daily driver, then straight pipe would probably not be in ur best interest....when i first got my exhaust i ran a couple weeks with a test pipe, resinator and muffler and it was horrible!(couldnt get the stock cat off the stock exhaust for a while...) it was so freakin loud and annoying, and if u drive by or anywhere near a cop, u'll def. get pulled over. if this is gena be ur race car, then go ahead and do it. as for piping size, 3" and up is for turbo so if u think a turbo will be in ur near future, get that, if not, go with 2.25.