Just have a custom system made. Back at college I had a guy with a Corsa Sport, headers and a few other goodies get rid of his exhaust for a custom CGS setup after hearing my truck. I sold it but still have my other V8 Mountaineer and just had a custom dual CGS system installed, nice and deep/agressive. Exactly what I wanted. Flows have a hollow/raspy sound IMO unless you have a nice cammed/built engine. I thought Magnaflow sounded great on the 6.0's though.
I had a Magnaflow system on my Avalanche and I loved the sound. It had a little drone but nothing that was annoying. Like it was said earlier the 07's have a pretty good exhaust note from the factory. I really wouldn't want it any louder than it already is. My plan is to see if I can get the muffler pushed further under the bumper so it is out of sight and get some chrome tips.
I'm sure Borla is working on a system for the 07. I heard the Borla split exit exhaust on my friends 03 and it really sounds great. I am getting one installed on my 05 along with a Volant air intake. I'm still young and I like making a lot of noise. Borla does that for me.