i thought about the 3 inch exhaust, but with the effects that it has on cooling exhaust temps and pulling air out of the manifold...I equate this to a 3 inch free-flow system in less over-kill.
I really believe you need some back-pressure to maintain higher torque numbers in the lower rpm ranges. Too little back-pressure might be more trouble than it's worth and potentially push all that useable hp and torque up the rpm scale in an unusable range.
It's a balancing effort, based on what your pushing in to get out...ie bigger bader turbo and the right plumbing. I'm still dynoing in different configurations as soon as I get more time.
I'd be interested in hearing your results with the 3 inch one on....i.e. a dyno when you get a chance.