magnaflow is coming out with system for 2012 part #15197 they have it on their main page but no spec or price. Probably around $800, somebody on forum say that a 2011 system will work but do not know what you might have to do. Might try custom exhaust.
Some of the other will chime in on flowmasters.
Go on youtube and type in Impala with flowmaster exhaust and magnaflow exhaust and see which sounds better to you. I personally love the magnaflow catback exhaust that I have. It sounds good, but not too loud in the vehicle.