Does 8k matter if I watch it in 6k

That 2880p for the files is the resolution of the flat 2D video. It does not 1:1 correspond to the resolution of whatever displays your headset has. That's not how you look at video in VR as the video is stretched into a kind of 3D sphere where you are sitting in the center of (and it's stretched in a kind of weird way so the edges are more squished together so it's less detail than dead center).

But that means in your headset you only see a portion of that entire 2880 video, you're effectively zoomed in to a part of it thus the actual resolution you see of the video is much less than that 2880p. And if you manually move forward to be closer or zoom in with the player controls it's even less. There is probably a point of where the ratio of file-resolution vs headset-resolution is high enough that you can't really see any visual improvement but unless you're using something like a mediocre mobile with GearVR or a really old headset, in most headsets 8K will have better details.

Higher resolution simply means more detail so you won't see the pixels as quick when it gets stretched out into that 3D sphere in VR. That said 8K at low bitrate or the same bitrate as if it was a 6K video might only be slightly better if at all. Because bitrate is equally important to have good details in video, there's no point in having high resolution with smudged out blurry details. That's why people have been asking for increased bitrates on SLR for 8K videos as those are suffering from low bitrate and in most cases that is causing a loss of detail.