The question is really how much more efficient can be gained beyond Maxwell (adjusted for process, of course.) For gaming workloads, I doubt there's going to be a huge additional jump, though I've been proven wrong about these kinds of claims in the past. So, for now, I'm assuming just process scaling in terms of perf/mm2. Say a 50 to 60% increase per mm2. That gives a 400mm2 chip roughly the same performance as a 600mm2. We already know that a 600mm2 doesn't strictly need HBM to be competitive...