What kind of horsepower gains could I expect from a 2013 ls SB 6.0? It's also ported and polished?

Are you planning on racing? Legally? Otherwise all you will gain is a destroyed transmission with NoX

Agree,and to add if rear end is standard and drive shaft not reinforced to handle it,you may throw them out on your first ride.

These guys are right. There is a whole lot more to going fast than an engine. Brakes, suspension, transmission and rear axle all require a lot of work ($$$$) to handle all that HP.

It's hard to guess-ta-mate, have it dyno tested. And it's the WHOLE package set-up! not just bolt-on nitrous.