tl;dr, 8K should have less SDE than 4K, 5K should have more SDE than 4K. But I doubt it’ll matter outside of very static scenes with bright backgrounds (e.g. Virtual Desktop).
Pimax 5K/8K:
Pimax 4K:
Pixels / degree (should be indicative of SDE, since all are RGB layouts):
But, even with the Rift I stopped noticing SDE in games with locomotion. It wasn’t until I tried to read text (Big Screen, Virtual Desktop) that it was a bad experience.
Edit: Note, this doesn’t account for screen utilization. 5K+ probably a little better. Maybe matches 4K, maybe not. (9% more PPD would have the 5K+ at 23.25 instead of 21.33)