stock exhaust is 2.25. Yes you can weld 2.5 mufflers to 2.25. Not really much of a performance gain when it comes to midpipe and mufflers its more of a sound improvement mod.
Yeah changing mufflers aren't gonna get you any power. That would be like blowing through a straw that gets slightly wider at the middle, the exhaust is going to have to compress back down into 2.25 piping after the muffler, so opening it a quarter of an inch for a foot isn't gonna do anything for you besides sound. Please stop typing in caps by the way.