ES3.1 is blazing fast and a way better than ES2, since like 4.21..
Did you publish a full game using ES3.1? I published my game called "StardustVR" on Go, and the difference between ES2 and ES3.1 was huge, ~ 10 fps gain from 50fps to 60fps using 4.21.
I was working a game for Go that was a lot more complex than what I see on the Stardust VR video. I didn't have any performance gains in ES2 and I used ES3.1 all the way. Do you use mobile HDR and PP at all in your game ?
On top of that Go will most likely die off soon anyway, so I don't even worry about that HMD and whether UE4 will support it on not.