So got around to confirming the 800mm 5.6 incompatibility with IBIS/VR. First I tried it on the D810 and it was again a dismal failure because the focus is so hard to nail on a DLSR (for me).
On the Z6 the focus works perfect half the time.
BUT if you turn IBIS on while on a tripod the picture s always a bit fuzzy. If you care just compare the two below at 100% on flickr.
Here at this magnification they look the same, this is the best I can get with IBIS, thought the lens was defective for a year or so, it is actually perfect, just so harrrddd to use.