Any time you present any kind of bend or something like going from a larger pipe to a smaller pipe,.. it creates a form of restriction. Its why I like sand bent or mandrel bend exhaust pipes. I dont like the kinked bends that many mass produces cat backs have. In your case.. I doubt it will kill any power that you can feel with a stock motor. It will sound a little different though,.. smaller pipes wont bellow like larger pipes do.

If you use 2 1/4" (2.25") tail pipes with the rest of the exhaust being 2 1/2" (2.5"),.. the whole exhaust may as well be 2.25". It can not flow any faster than the smallest portion of the exhaust. In doing so you relegate any 2.5" pipe useless.

Hurst