I have gone with a 2.5" after the Y pipe with a simple straight through muffler. This and my CAI are designed for the 2500-3500 rpm range in mind. I you want just all out top end power, go for a 3".
I can attest though, getting rid of all that resonator and muffler from factory makes a significant improvement in performance.
I am finding, however, you can keep your foot in it to those rpm ranges and actuallyget better mileage than if you baby it all day long.
It's all about how you design and drive the truck.