On my S4, I had Y-pipe cutouts with caps that I manually removed and added turn-down tips, bypassing both mufflers and a lot of tubing. I saw about +40wHP doing this the way my car was configured.

On the 996TT, it would seem easy to run a pipe bypassing the muffler with an automatic butterfly valve in place dictating which path for exhaust flow to travel. Has anyone done such? Photos?







Dyno'd back to back after opening dumps:

 

dont be worry about me, but this y-hose design doesnt be a free flow system if its closed......with closed dumps the oem system looks better for flow....so you loose power on this way