sorry but the people who buy merc, bmw, lexus, infiniti or audi won't even know or care the g8 exists. drive down any rich neighbourhood and count the ratio of imports to "american". i would say it is 100 to 1. the g8 has zero prestige for the well heeled crowd and the regular joe won't be able to afford the v8 sucking all those extra dollars out of his pocket. so who is going to buy this car? gm should offer it with a variety of engines sizes like bmw does and see what happens.
I only mentioned Infiniti, and there's a reason for that. Not everyone who buys an Infiniti G35 is a luxury-loving blue-blood. I've met folks in Infinitis who are really just boy racers who wanted a step up from a sport compact, not your typical Merc-buying luxury crowd.
For someone who wants RWD, four doors, sharp handling, and great power for US$30k and change, the G35 and the G8 are the two cars to test-drive. Though the Charger RT and Maxima could also be painted as competitors on different levels.
EDIT: Don't get me wrong, the G35 also does pander to the luxury crowd, but it kind of plays to two different crowds simultaneously. Cars like the C-Class, on the other hand, ONLY pander to the luxury crowd.