I haven't actually received my GFX100S yet. How's the rolling shutter on the GFX100?

Yeah, I have the Rokinon 24mm, 35mm, 50mm, 85mm, and 135mm cine lenses in Nikon mount. 50mm and under vignettes for 4:5 stills, but probably wouldn't at 16:9. I have the Mitakon 65mm 1.4 and the 85mm 1.2, but the 85mm vignettes like crazy. The Rokinon 135mm is an amazing lens on the GFX. I also have Pentax 645 lenses in 35 and 55mm with a tilt/shift adapter. I'll probably do more video with the GFX100S, but the 50S is terrible at it.

Ok, but lens compression as a concept has nothing to do with sensor size. The depth of field does, but outside the Mitakon and some longer lenses built for 35mm systems, there aren't many fast lenses available for GF.

Lens compression is a myth. Field of view and distance from the subject is all that matters for perspective. I own a GFX 50S and 100S, and while a larger sensor offers benefits, lens compression is not one of them. And the GFX100 is not a DSLR.

What the hell is the mirror for?

Gaffer's tape is not duct tape. I do not recommend you use them interchangeably.