Well, if you play around with supersampling on any given current HMD, you will realize that it makes quite a big difference in terms of perceived sharpness.
On the 8K, of course a shitty upscaler could result in no or even detrimental effect, but what I saw in the hands-on I had at the VR Days in Amsterdam last autumn was actually pretty good, I did not notice any significant aliasing.
And the other point, where the upscaler does not matter, is the pure pixel density of the physical displays, and the spacing between them. Even on the brightly coloured, even planes in Fruit Ninja I really had to look very close to see the actual pixels, and as soon as I took my concentration off that, I would not see/notice them any longer. That too is a real improvement which the Vive Pro screens might not have, surely not to the same degree (I have not seen anything being reported on that subject on the Vive Pro yet, we will have to wait).
And then there is the sweet spot, which is really pretty narrow on the Vive, while the 8K practically is sharp for 95% of the screen, only at the very edges it becomes a bit blurry. Have not heard that Vive Pro addresses that point, which is astonishing given their teaser strongly seemed to suggest they resolved this shortcoming of the Vive.
So in terms of visual quality the Pimax has a good chance to really be a leap over the Vive and Vive Pro. But let's wait and see, what the hands-on's of the coming days reveal !