The G8 GXP only gets 20mpg highway and is rewarded with a GG tax for it. Yeah, there's a rebate, but who is to say the Camaro won't get a rebate in a year. I'll take the Camaro with its 24 or 25mpg with almost exact performance. As you add power whether through bolt on or forced inductions, those 3.73 gears might give you trouble off the line in the form of tire smoke.
I should mention that I really like the GXP, just too much money and I wanted better gas mileage.