I managed to find a store which had the MB-D18 in stock and purchased it.
I find the grip very well made and makes a solid joint with my D850. There is no flex whatsover between the two in my copies of camera and grip. I'm happy with the shape of the vertical grip though I find the main grip on the camera itself to be too narrow and deep for my fingers.
I read that there may be flex between the two but this was not experienced in my samples. Perhaps someone did not tighten the joint properly, or there could be some small changes that Nikon may have made to the grip or camera to make a better fit (which would explain the delay in availability, if true). It feels like one piece when mounted and tightened.
I can start to look at the possible effects of the EN-EL18a battery (I don't have the b) on the AF performance, if any. Shorter mirror blackout time should improve tracking.
Tried CH, release priority, flicker reduction off, 14-bit lossless, I got 28-29 shots at 9fps before slowing down (the menu guide says buffer should give 29). I used an EN-EL18a from my D5. However, my image was underexposed to get a 1/2000s shutter speed, and I believe the buffer may fill up faster when doing normal exposures. I will try it outdoors during the weekend.
I didn't find the sound too bad for a high fps sequence.