It depends on your hardware. Normally you should see better an 8K VR video with less bitrate than a 5K one with more bitrate, because the 8K one has more resolution, and in VR the resolution is more important than the bitrate, since the lenses magnify the pixels. The more pixels there are, the sharper it will look. With fewer pixels it looks a bit blurrier.
But as I say, it also depends on your hardware. In Quest it seems that the difference is less noticeable, I use PC VR where I apply 2X supersampling and MSAA filter even to 8K videos, and I see more defined 8K than 5K, even if they have a crappy bitrate as JAV 8K VR has.
The 8K VR videos that are being uploaded take up less space than the 4K and 5K versions because they have very low bitrate. They are a big mess. I don't know who is to blame, Fanza or the uploaders.
The best thing to do is to download both versions, and compare them yourself.
That
Sakura Momo video lacks one thing to be perfect: face-to-face missionary. I love
Kinoshita Himari, but I'm not going to download that video because of what you already know...