It means that via dyno testing, This mag (and quite a few others, too) found the LS3's ACTUAL power output to be ~10hp higher than what GM says it is.
GM has a history of underrating their engines.
But that's not the big deal. It's how responsive the engine is to very minor modifications. This tester managed to hoax an extra ~30 hp with just an intake alteration and installing an electric water pump.
I think that's pretty impressive. It's also been said that the L76 (most likely engine for Camaro) is even more responsive to mods than this.
