I've read quite a bit about how the A7RII shows thermal noise at longer exposure times. So, I headed to Joshua Tree with a couple of wide angle lenses for some milky way shots (my first ever.) Went fairly well. Below are some botched raws (badly framed, perhaps underexposed, etc.) that may nevertheless be useful for reviewing noise performance.
FE 28mm at f2, 15 seconds, ISO 3200. Just the milky way:
No bulb, single shot mode, LENR was off, manual mode. No dark frame averaging (too lazy.) My overall impression, even on underexposed shots: better than I was expecting. I processed using C1, default sharpening and noise reduction settings. There were very few warm/hot pixels that I could see in the starfields, even when exposure was raised. In dark landscape areas, there were certainly some red/blue warm pixels when exposure was raised, but not a profusion of them. Quite fixable IMHO. The Rokinon was beautiful to use because of the hard stop infinity. I had more trouble focusing the FE 28mm f2.
I understand that this addresses only a narrow subset of astrophotography--single 15-30 second exposures. I don't see myself getting into the heavy duty, super long equatorial mount exposures, or stacking 75 exposures over an hour or whatever. So for my purposes, quite acceptable performance, given my somewhat peripheral interest in this genre. If you do primarily astrophotography, it may not be the best choice based on what others have reported.