What methods/technologies to reduce required performance for virtual reality are there?
Avoid stereo when possible
There was some research published recently measuring when users can or cannot tell whether the specular contribution, which is view point dependent, is different between eyes.
Perception of Highlight Disparity at a Distance in Consumer Head-Mounted Displays
In the future, this could be use to share some computation between the two eye renders. Unfortunately this sounds hard to take advantage of in a typical rasterization pipeline.
Some games already avoid stereo when rendering far objects and background.

Reduce shading when possible
Lens distortion
Part of the NVidia VRWorks SDK is a feature, Multi-Res Shading, that allow to reduce the amount of shading, to take into account the fact that some pixels contribute less to the final image due to lens distortion.
In Alex Vlachos' GDC 2015 presentation, he also mentions the idea of rendering the image in two parts of different resolution, to achieve the same goal.
Foveated rendering
Finally, like your question mentions, foveated rendering aims at scaling the resolution to take into account the fact that eyes have more precision in the fovea and less in the periphery. There is some research on the topic, but this requires eye tracking like in the Fove HMD.