Catback systems are legal in most areas, and also, shouldn't void the rest of the car warranty (but included in the warranty if you did an LPO exhaust). Besides, I haven't heard of very many exhaust warranty claims from behind the cats.
I tend to look at it as not adding HP/TQ. They're just letting some more of the pent-up HP/TQ out as it's already in there. Add a better flowing intake system, and you'll do the engine proud. Allowing the LS engines to breathe is very beneficial to performance. Just like a marathon runner, you would expect them to lose a bit of performance if they were constipated and then clip a clothespin on their nose to boot.
2010 2SS/RS Auto, Sunroof, Engine Cover. Transformer Edition package.
2013 ZL1 ragtop #5478. 1 of 54 CRT M6 'verts.