4K or higher videos with the Pimax 8k

you would play the highest that your pc and video card can handle. I watch 8k with my index and g2.
I did have an 8kx but returned it due to some issues. overall it has a great fov but difficult to focus on the whole screen. I'm actually happy with my index despite the lower specs, it just works without headaches and really it does a great job. the g2 is a bit more awkward for me with windows reality and those crappy controllers but the image is great. downside is the FOV unless you modify the mask to put your eyes closer.

Full disclosure: I do not own a Pimax 8k X, but can chime in purely on the VR aspects of your questions. Also I was bored so here's the novel...

If your PC can run the SLR app or Deo VR through Steam and your headset can operate Steam, then the headset itself should have no issues whatsoever with it, whether it's a Pimax 8K X or an Oculus Rift (anything as long as it's pc-based). There's other methods for specific headsets (Quest 2, any Windows Mixed Reality headsets), but my understanding is that the Pimax can operate through Steam.

1.) As a general rule, always always always go for the higher resolution and larger size (more importantly higher bitrate) versions of the files, especially to take advantage of the resolution of the Pimax. Think of VR as just being a video projected onto a massive curved screen you're sitting in the middle of. The resolution of the file is the quality of the image being projected, and the dual 4k lenses in the Pimax just means you'll be able to take full advantage of the clarity of the larger resolution files. In general, it 100% most definitely makes a difference going above 4k (insert rant about misc. studios "upscaling" their videos, studios loosely using the term 6k/7k/8k, etc.). But tl;dr yes, bigger resolution and bitrate/file size is extremely important and worthwhile.

2.) I've tried downloading a few of the 120fps scenes available on SLR, and tbh I did not notice much of a difference at all on my Index. A bit smoother was about it. Then again, I could be wrong but I don't think there has really been any native 120fps releases from basically any studios, just interpolated ones.

3.) Two parts to this: the first is that there are many different slightly-different sources on this, but when you're looking straight ahead the human eye can see just shy of 180 degrees, it's only when you look out the corner of your eyes that your vision expands a bit past that 180. The other part of this is that when you need to focus or look at something in your immediate surroundings IRL, you almost certainly turn your head along with your eyes to do it. For example- if you're sitting down looking at your computer screen as you read this right now, you're probably not picking up the details of the wall or pictures in the background, the cup on your desk, that freckle on your mouse hand, or whatever else is in the periphery of your vision (though you probably are now that it was mentioned haha). I mean, just make a literal circle shape out of your fingers/hands and hold it a varying distances (we're talking cm's/inches) from your eyes: my Index is around 130 degrees max, the Quest 2 is a good deal less than that so both of those are a much smaller "circle", but the Pimax 8k X with 200 degree fov is basically like having no "finger circle" in front of your eyes at all, it's basically just your regular vision.

All this is to say: I genuinely don't know much about the 8K X other than what I've read on the various forums, which is that it's fantastic quality visuals but takes a good amount of work and tweaking to get working to it's full potential. Will also obviously require a high end video card, and will have to judge whether the price point is worth it to you personally, etc.

Just a point of clarity an 8k video is not 8k per eye, it is 8k total and split in half

Johnny166 Nah, you shouldn't have any issues at all running even the highest quality that's out right now with those components. The only caveat will be: if/when you've got whatever headset you choose, look up the forum posts about the required codecs and whatnot Windows requires to smoothly play these files. Otherwise it'll be chop-city.

I use DeoVR through Steam as my go-to, but it's functionally the exact same app as the SLR app, so either one works.

For FOV: human eyes might have a pretty damn wide FOV in theory, but in practice we tend to be more tunnel-visioned on the area around our immediate focus. If you do a 10-second experiment you'll see what I mean: pick a point and look straight ahead. Then put up one finger and hold your hand a foot or two away from your head but outside your vision to the right side. Still looking at that point straight ahead, slowly move your hand/finger from the right towards where your vision encompasses and hold it in place the second you can see it in your peripheral vision. Keep your head still, but take your eyes off that fixed point straight ahead and try to look at your finger. You'll now see there's like another 20 degrees or so you can see past what you were seeing before, everything past your finger. This is basically just about where the black bar would be on the Pimax.

And again, since basically 99.9% of the action in VR porn occurs right in front of the viewer, that means that most head movements you naturally do to look around will also be primarily small ones in the center of the video.

So to sum it all up: I genuinely don't think you would virtually ever notice the black bars with a 200 degree FOV. I noticed them with my old Oculus Go (roughly 100 degree FOV) and they bothered me, but rarely notice them with my Index (130 degree FOV). But again, I can't really speak to any specific headsets except the Go and Index.