The turbo is the back PSI in the system anything after needs to be as free flowing as possible.
When purchasing my exhaust I really researched and what I came up with was that the extra wieght in the dual 3" system was not worth it. Check out the weight differences and remember a 100 lbs is a typically worth a full tenth gain in the quarter. You want faster
Stick with the dual 2.5 (stay away from the 2.25 flowmaster system !), 3" ATR crossflow (I am super happy with) or the 3" single shots( pretty loud though) that are out there. MANDREL BENT TUBING IS A MUST stainless if possible! It is expensive but try to scoop up a 3" DP while your at it.
As far as TQ numbers falling off....The TQ will increase due to the freed up exhaust flow.