500mm^2 is only 14% more than the 290x so I wouldn't expect miracles. Even 550mm^2 would only be 25% more. If that equated directly to a 25% performance increase then it would be the fastest GPU out there but not by a huge margin.
For what it's worth I've seen rumors saying it's 600mm². Plus, perf/mm² improvements are always possible, just ask Maxwell; or RV770.
I'm not saying I expect this but it could happen.