Just got my GR axle back yesterday with 5" mufflers and quad staggered single-walled tips. I couldn't wait and installed it when I got home from work. AHHHH the bassy drone.
Everything else is perfect. But, anything under 3k rpms sounds like I'm riding in a helicopter filled with 12" subs. I guess it's to be expected in the hatchback though. So, I just ordered some http://www.amazon.com/Stinger-RKCP12-RoadKill-Carpet-Pad/dp/B001TH8OC4 to put down under the spare and I think that will really make it perfect. The sound is great. I just need to cut down on the bass.