The diameter of the muffler and associated tubing will only have an effect on the max hp you will achieve at top revs. Maximum torque is pretty much set by the camshafts and fuel type (with upgraded ECU), and being in the mid range of engine revs will not be affected by exhaust diameter/flow rates.

2002 Sporty , Ironman 2.5" spring, 2" body, 15 x 7 ROH wheels, K&N, 15 x 10.5 Simex Centipedes, Powerchip 91.

"Never argue with an idiot. They will bring you down to their level then beat you with experience!"