The difference is in the LS motors. On the outside, yes everything looks ordinary between the LS*'s BUT it's what's inside.

The LS1 (5.7) output is 300 - 325hp (year/vehicle model dependent) the LS2 (6.0) in the GTO is 400.

To get 360 (to the wheels) in a LS1 car, you'd need full bolt on AND cam swap.