Here's a thread off their board. New Best 12.59 and Challenger SRT8 Kill - Pontiac G8 Forum - G8Board.com
That guy has the numbers to win.
Heads/cam/Long tubes in a G8 will pretty much kill an SRT8, but stock, the G8 is slower.
A stock G8 is actually slower than stock Cobalt SS in a 1/2 mile drag race.
A G8 will definitely be cheaper to buy and build than a challenger. Even a Vette would probably be cheaper on an ET to ET comparison.