i have recently taken off my cat back exhaust as i was driving a bit of a road trip. I noticed quite a bit of power loss all through the range.
and Perhaps a week ago, i put it back on, and damn there is power again, noticable low and high in my rev range, but definately more at top end.
Mine is 2 3/4", and its drony in certain RPMs (mainly at 110 on motorway), most of the time, its sexy. Get a big resonator, big as possible imo.