I've had the truck (with same exhaust on it) for 5yrs, and it has 3.5" tips on it now. i've always wanted it to be louder but couldn't justify spending 450-500.00 on exhaust that there was nothing structuraly wrong with. but now the exhaust hangers have rusted and broken loose of the pipe, causing a hole. so now i can justify getting the louder exhaust i've wanted.
another guy where i work had the 2-in/2-out flowmaster 40's put on his truck with 2.5" pipe and 3.5" tips about 3 months ago, and it sounds decent (slightly louder than mine) but it's still isn't very loud. i would be aggrivated if mine came out sounding like his, he even wishes his was louder.
I seriously think that the fact that these trucks have a dual resonators, and dual catalytic converters makes them a bit quiter from the start