I have all the AFS masts, and agree with Bryan, the skinny masts are awesome for maneuverability and add a bit more glide (and presumably speed).
The 75 is the same, just changed name to be in the skinny line. I believe the construction of the 85 skinny (and probably 75) is different to standard UHM in that it is full carbon rather than having a core of some kind, and is more expensive for that reason.
AFS don't recommend using skinny masts with the Ultra foil (not just due to span, but something about tips not being swept back), but it seems some people find it fine.
I find the 85 skinny to have a bit of hesitation / feels uneasy when pushing hard on the 1050 + ultra, a tiny bit on the 850 and is a dream on the 650. It's mainly notable on flat water when I can turn the foils as hard as possible, but is fine in waves & down winding where I can't rip quite that much. The difference in turning and glide is definitely noticeable (more so than the 75, where I'm not sure how much is just due to riding a shorter mast than usual).