Depends on the game, your settings etc. At 3K resolutions you probably will be GPU limited in most cases, but that is really just a guess. There just is no clear and easy answer to this question. 3K is a bit of overkill in a VR headset BTW. Certainly will look great due to the supersampling, but in most cases it will make sense to use a lower res with better performance.
It’s also worth to note that vorpX uses a multithreaded rendering approach, which costs about 10-15% overall performance on top of the G3D performance impact, but on the other hand makes low framerate situations (<40) more tolarable. Such situations typically aren't handled well by the headset runtimes.