There isn't a benefit yet, thats what I'm trying to tell you. lol The Technology is not ready yet. Variable eye sizes, the shape of the socket, the depth level the eye sits at in the socket, all has an impact on the effectiveness of eye trackers even at the highest end.
Even if they get the hardware down, they would have to redesign the entire graphic pipeline to be able fully utilize the benefits of foveated rendering.
"If you can render half of the screen in high quality and the other half in low quality, then that's going to add up to really important speed gains."
We already do stuff like this with lens matched shading and the other tools in the VRworks suite. You dont need eye tracking at all to do that.