The G8 GTs run high 13s bone stock, but it takes nothing more than a CAI and a tune to run around 13 flat. That's a hell of a gain, and suggests that the GT is basically a GTO with bottled up performance from the factory HP-wise.
It always does amuse me though that the only thing people are concerned about with the G8 is that it can be fairly quick. Even if the thing came with a freaking 3800, I'd still take a G8 over a GP because it is world's better in every other aspect. The styling will be subjective, but save for the GTO, the combination of the braking, handling, ride, fit, finish and styling of the interior, materials used, RWD, etc. is a night and day difference from anything released under the Pontiac make in a very, very long time, let alone a GP. It's just an added bonus that you can get a decent power plant with them.