The funny thing is it's a 4.3" display, which with just one is not large enough for a wide FOV HMD, plus no IPD adjustment so seems like very little chance of them using just one of these displays in an HMD. It is however a good size to deliver a 130deg+ FOV if you have one per eye (the current 100 deg FOV HMD's use a 3.5" display per eye).
So it's actually going to be two displays... 2 x 5500 x 3000, which is about 33 megapixels all combined. Which is very close to the pixel count of
four 4K displays
Though, reading about it in more details, the display has been directly developed with in line foveation integrated circuitry, and Google recently published their latest work on foveated rendering which at the bottom alluded to "Foveated rendering based HMDs being on the market within the next 1 to 2 years". Combine those two facts and it is a practical certainty that they are planning on releasing an HMD with eye tracking for foveated rendering. From everything i've seen you can realistically expect a minimum 2x fold performance increase, with up to 10x with the technology perfected.
So give it two years for the release, GTX 1180's to be on the market.. and you could feasibly push certain graphically simple games at those pixel counts and refresh rates. Also worth noting that just because the display is 5500 x 3000, they do not need to render at that resolution. Even if they rendered at half that resolution and did pixel doubling on each axis with a bit of anti alias, you'd benefit from massively reduced screen door effect, in fact it is going to be basically invisible at that high of a PPI, and still much better visual quality than current Rift or Vive. Then in the next few years they can slowly work the way up to native resolution and beyond with super sampling as GPUs catch up and the performance benefit of foveation gets greater.