Backpressure is not a good thing. When people say they feel or don't feel backpressure, they're really feeling the lack or presence of exhaust gas velocity. Kind of like if you blow through a straw then blow through a toiletpaper roll. It's not that the straw has more backpressure, just higher air velocity. Velocity, good, backpressure, bad. Backpressure would be created by clogged cats or a poorly flowing muffler.

Depending on your desired rpm, your pipe diameter will vary. The explaination above is for a street-driven car, like I said. If you turn your X into a track-only, 10 second, iron block 454 w/ a 300 shot, then of course your desired diameter will be different.