Given that one of the biggest selling points GM is pushing on these things is fuel economy, I'm not sure they are in a rush to get a V8 engine option out there. This is just a guess on my part, but it would somewhat undermine their marketing angle. They want these Lambdas to be seen as upscale full size (7/8 passenger) SUV/Minivan alternatives that beat all competition in MPG. That would no longer be the case if you put a V8 in it.