To build on what Chris said, I would not open up the exhaust too much. Yes 1hp here and there adds up but unless you're willing to rev high you won't be able to use it. Most HP numbers given on performance parts is recorded at their sweet spot and not across the board.
Also, as you increase HP, you typically loose low-end torque. Case in point. I noticed a big difference in high-end horsepower when I punched a 1x2" hole into my exhaust. Low-end torque was mediocure but there was noticably more power the higher the engine was reved.
Fixed the hole and the ending felt locked at the higher RPM's but low to mid torque increased.
I've mentioned this on the list, but I don't think I've mentioned it here. I'd like to see a two stage muffler. This would give you low to mids at lower RPM's but once you start climbing over, say 3000 rpm, it would open the second stage which would make the muffler flow increase and boost the top end, when you actually need it.
Two-stage exhausts are actualyl used on many vehicles. Another reason for the two-state is to reduce noise at low speeds. Lexus, Aston Martin, Mitsubishi, etc... are only a few of the cars that use them.
In any case it's an interesting idea to get the best of both worlds.