Warspeed says 2.25" but some people on the org say they put 2.5"-
Anyone can help with this one? And also is there a diff. performance
wize w/resonator compared to without resonator(besides sound?
Help greatly appreciated-

You won't notice any difference with the stock setup. I bought the 2.25 because it was in stock. The 2.5 doesn't provide any benefit unless your boosted.