Our testing has shown the following:
Downpipe + OEM cat-back = faster spool, marginal increase in hp, most of the torque increases
Full Exhaust = faster spool, healthy increase in hp and trq
2.5" Full exhaust = more torque, less hp in the higher rpm's
3" Full exhaust = less torque, more hp in the higher rpm's
My suggestion is that the 2.5" full exhaust is the best all around performer for daily driven vehicles and road courses. The full 3" exhaust is perfect for those concerned only with top end performance like in a drag race.
However, if you plan on a turbo upgrade later down the road, you'll want the 3" as a 2.5" is too small for most big turbo applications.