I put a pair of Flowmaster 50 (Delta Flow) mufflers on the GXP at 30k miles. Awesome sound. With the OEM GMPD mufflers, the sound was good at half to full throttle. However, the entertainment value of draining the tank to enjoy the bellow produced a considerable negative cash flow!
The Flowmasters entertain me simply rowing the gears up to 2000 RPM, but can induce whiplashed toll booth attendents upon launch. They sound muscular at idle and low RPM, and explode to roar with mashing the right foot.