AMD has quad core jaguar cores running 2.0+ ghz on 28nm with lower spec gpu modules. It's what my mame machine runs and its sic for that task.
If they just do an optical shrink 2.0+ octocores should be easy in large quantities. Microsoft could use the existing cooling setup on that chip even with a 380+ class 14mm part and possibly get 2.4ghz per core with slightly lower yields.
I think this is what we will see transpire. And honestly I am warming up to it more and more. The big issue is that this is uncharted territory on the 14ish nm process so current tappings for dev kits for both Sony and Microsoft probably suck power and run hot and require several more respIns before they get the chips perfected.
Which is why I think 2.4ghz is the max jaguar and a 380x class video despite core generation is more or less what we will get imo.
Anything else is icing on the cake. And if MS does throw gauntlets and puts down zen cores then sign me up x 2.