I'm not 100% on how the test works, but as I understand it you have a window (no more than a couple percent) that you can pick your advertised power from. I imagine that the LS7 was rated as conservatively as possible with the SAE test procedure, since I'm pretty sure it was the first engine that was certified with that procedure. If all that is true (which it might not be), they would have enough leeway to rate it at 515 or so without having to actually recertify it. I don't know if SAE allows such a thing to happen, but it is a potential loophole they could exploit.
Note, if I've gotten any facts wrong in the above, just ignore any points I made with them
don't believe a thing you read about the next gen Camaro -- as history has proven time and time again:
WE DO NOT TALK ABOUT FUTURE PRODUCT PLANS PERIOD FbodFather
Camaro5 Fest sub-forum