Another 2.5 inch vs 3.0 inch?

You definitely don't want to go from 3.5" to 2.5" with your collectors. Here are high quality 304 stainless 3.5" to 3" collectors. Pypes Stainless Steel Collector Reducers PVR13S

You've chosen some really nice high flowing headers. The 3" X is rated to 550hp and by the time the exhaust is through the mufflers, you've taken most of the back pressure away from the heads for maximum hp. In theory, if you could figure out a way to get 3" tailpipes to fit in there, you'd probably get another 10 hp? With mandrel bends, it's not about the size as much as it is about the smooth and perfectly round tubing.

The 3" to 2.5" hybrid system is perfect for your application unless you want to run dumps and don't mind the dust kicking up and the very loud sound.