If all your looking for is HP, and good times at the track, another alternative could be an exhaust cutout. Place it right after the Ypipe from the headers.
You can have a nice quiet system when your cruising, and a free flowing (and loud) one when your racing.

Definatly a worth a lot more $/hp over duals.

 

unless you run dual 3" all the way to the back, the best thing that you could do is run the already mentioned 4" single exhaust. i know that this is not really what you want to do, but by areas of the pipes the 4" system has more area than any dual setup of 2.75" and smaller. 2.75" duals have a cross-sectional area of 11.88 sq.in. where the 4" single has cross-sectional area of 12.57 sq.in. and 3" duals have a cross-sectional area of 14.14 sq.in. so yes a dual 3" setup is probably what you would want to run turbo(s). just thought the facts and figures would be nice for you to have.