Got a new 09 LMM. Want to get rid of the snorkel. Does anybody have any experiance with the Banks systems? Will just a cat back help or hurt performance?

I don't think Banks makes anything other than a DPF back exhaust for the LMM. It likely makes zero difference in performance, but (IMO) it looks 1000X better than the stock exhaust. I have the single outlet Banks exhaust, and you can see pictures in my garage.

I would suggest doing a full exhaust if you want to see any mileage or performance gain. However in order to do this and be code free you will need a tuner. This can also void your warranty, but your dealer plays a large part in that.

Ummm, "it's up to the dealer" that is now a fallacy (based on my experience)...
Took my 08 LMM in for a warranty issue two weeks ago - service of the electrical harness on the tranny...

Remember, THEY sent me the postcard telling me to bring it in...

The service manager apologized saying that he had to first take a 'shot' of some page on the diagnostic screen and send it to GM for approval of the warranty repair - again, I am waving the postcard they sent me saying 'bring it in'... Took about 10 minutes for them to get the approval back... Based on that I suspect the dealers have very little to say what gets done under warranty...