Mapillary currently have a fixed 2K max resolution for the projected image
Where did you get this from? Mapillary’s resolution limit is 108,000,000 pixels.
I agree that the 8K resolution is a bit of a limitation OSM mapping
I did not say this. All I have said is that there is a disparity between resolution and encoding bit rate, which causes the final image quality to suffer so much that it becomes useless for mapping. In fact, I have deemed 8K sufficient for mapping. Because the reason for such a camera to exist and be useful for mapping is that it should deliver data that enables to extract information which goes beyond what you can extract from satellite or aerial imagery. Otherwise all of this street‑level imagery capturing exercise is pointless.
For the next major step in quality I think around 24~32K (and 6+ 4K cameras) are needed, and this implies a significant increase in physical size, storage space and cost.
Certainly, higher resolutions would be nice to have but there is obviously an economic trade off here. It is way easier and cheaper to tune bit rates and design storage throughput correctly than to produce higher resolution photoelectric sensors.
AFAIK there is a upper limit for the useful the camera resolution.
Yes, but this has nothing to do with Mapillary. Google’s research in street‑level imagery capturing has concluded that 100 to 150 MP constitutes a natural upper limit for 360° panos. You gain little to no additional information from greater than 100 MP 360° panos.
When the camera is at stand still the artifact noise lessons. However, it is still visible with the naked eye. 
It is a shameful waste of the Theta X’s high quality sensor(s) and optics.