W/ re to backpressure. . .the larger you go w/ the diameter of the exhaust pipe the more you increase your top end and the more you increase your low end. Backpressure helps greatly w/ your 0-60 performance while a larger diameter pipe help w/ lets say your 'cruising' speeds. I have had technicians tell me that a 2.5" pipe is probably the largest one should go on our 3.3L engine set-ups. A larger pipe diameter is more at home on a big block V8. The interior resonance of my 40 Series is not as bad as I thought it might be. Under hard acceleration its quite evident but under cruising speeds its not very noticeable. My current exhaust pipe is a 2.25" so w/ a 2.5" pipe it might be a bit louder.

Hope this helps a bit.