Gaming rmbp external monitor

[bohnje]

How much better would the rmbp be able to run games on a 1080p external over its own high res screen?

 

[Stetrain]

Well 1080p on the internal screen and 1080p on an external screen should put the same load on the GPU, but it will probably look better on the external display since it will be at native resolution.

As for specific performance predictions that would depend on the game. You should be able to look up benchmarks for the GT650M to get a feel for it.