Typically speaking, how much of a difference is there between the stock headpipe with quality slip ons (like Rienhart or V.H. among many others) and a 2 into 1 like thunderheader or V.H. propipe. I'm not asking about true duals. I find it strange the aftermarket by and large seems to have ignored the benefit of a balance pipe if your running duals. Only exception is the new V.H. headpipe which from my understanding only works on 09 and 10's.

 

Are you talking difference that an experienced tuner would see while dyno tuning it, or a difference that you as a rider will feel?