Finally got the screen yesterday. It takes them a few days to make it, then obviously the shipping from Taiwan. So, it was about two weeks total from when I ordered it to when it showed up.
My first impressions are very good. I put the screen on and mounted my 50mm 1.2L to it, and what's seen in the viewfinder looks very similar to the actual photos taken at f1.2.
I did some experimenting using the DOF preview button on the camera, and my initial impressions lead me to believe that it changes the maximum aperture you can see through the viewfinder to about f1.4. And by that, I mean that if I look through the viewfinder and stop the lens down, I see pretty much no difference between f1.2 and f1.4, but then as I go to f1.6 and everything beyond, the viewfinder gets darker, and the out of focus areas get sharper. This is contrasted with the original screen that wouldn't show any difference until about f2.8.
As far as metering goes, I'm able to get a good impression of how far this gets thrown off, because I have another 5D4 with the original screen still installed. If I mount my 50 1.2L to both of them, set the shutter speed and aperture the same, point each camera at the same scene, it seems like the metering on the camera with the modified screen is around 1/3 stop off from the camera with the original screen. The camera with the original screen wanted to use ISO 500, whereas with the same settings pointed at the same scene, the camera with the new screen wanted to use ISO 640. What did surprise me is that when I ran the same test with my 11-24, which is an f4 lens, the two cameras still seemed to be only 1/3 stop off from each other. The viewfinder really is not that dark, even with an f4 lens.
As far as the quality of the screen itself, I'm very impressed. It does not look like it was crudely trimmed down from a larger screen. The screen itself comes in the original Canon packaging, with some printed stickers on it to indicate that it's actually made for a 5D IV. I felt a little cheated when I realized that all they're doing is cutting down larger Canon screens, but it's really good work. I don't think I have the tools or the skill to do whatever they do to make these screens. It's pretty much indistinguishable from the factory screen.
I'll try to remember to update this with more impressions once I use the camera with the new screen, but I'm pretty happy so far. It's actually really cool to sit there with an f1.2 lens and manually focus back and forth and watch everything in the foreground and background of the scene go from sharp to way out of focus right there in the viewfinder. It's really nice to be able to see what your photo will actually look like with a fast lens right in the viewfinder.