They started shoving the LS2 in the GTO in 2005, following its introduction in the Corvette. According to the Wiki, you guys had the L76 in the VZ, not the LS2. They're related of course, but had a little less power and was related more to the LQ9 than anything else. That, as a matter of fact, has carried over to the VE and indeed is the V8 we find in our G8.
I don't think they'd overshoot the 550 BHP standard set by the CTS-V because of that. Given that the Pontiac and the Cadillac weigh about the same (give or take about 200 lbs), the performance would be too close and the price difference wouldn't be enough to justify the Cadillac option. Using the LS7, keeping the power down to 505 BHP, would keep the distance... Well, at least one would think that.
Depends, but yes, I think you're about right. The G8 GT is a bit faster than the 535i, thanks to the extra 60 BHP, but it wouldn't take that much more than 500 BHP to keep up with the M5. The 414 BHP Clubsport R8/VXR8 does just fine, and thats down 100 BHP. So I guess we just have to be happy with the LS3 in the end?