No HP gain from mufflers. Stock 6i exhaust is non restrictive. (Some areas for improvement, but not major ones). 6i has 4-2-1 headers, going to a single pipe/dual cat, and then out to two pipes...each muffler is handling only half the exhaust load, barely any restriction at the mufflers at all.
For the 6s, its going from restrictive manifolds, into big precats, into two main cats, back into a single pipe, then back into dual pipes and dual mufflers at the end. Once again, exhaust load split between two mufflers, not a serious restriction by any means.
Honestly if you wanted the most performance possible out an exhaust, you'd get a single pipe, with a SINGLE muffler, going out. Eliminate the second muffler, the Y intersection, and the corresponding pipe. That'll increase the efficency of the system, increase flow, and decrease cost and weight. Of course it will look debatedly less cool.
