I do not believe you will get any significant measurable increase in performance from switching the 2011 mufflers from stock. You will change the sound but might actually loose some power. These 5th gen Camaro exhausts were heavily engineered for sound and low restriction. The "mufflers" are like empty metal suitcases inside.
Removing the cats might help power if you take them off. Car will smell worse but maybe that is not an issue for you?
2010 Camaro SS 6speed, Supercharged, LS-9 cam, custom tune, stock exhaust.