I love my 5.5 I have to say, and in the wagon version, you hear the exhaust inside the cabin really clearly, so no need to mod the exhaust. I have had mine about 14 months and I've only had one problem, which was squeeky steering at crawl speed. They replaced two active engine mounts under warranty.
I'd recommend buying via a dealer, I got a two year warranty, and a nice welcome to amg letter plus AMG usb keyring.
On the mpg front, I have squeezed around 28mpg out of mine, but I average around 19 normally.
I was also surprised to be told my pads are only 50% worn after 14 months of ownership. My local merc dealer told me I was obviously not driving it hard enough lol. I think I do!?