For fast wrist stuff, string changes come from sort of a “sawing” motion from the shoulder, accompanied by a bit of an extension of the range of the wrist motion as I grab the next string. For something like the Paul Gilbert lick, I just grab the one note on the adjacent string with a bit of a jerky extension of the wrist motion.
Motion smallness on the wrist stuff and the MAB stuff is a side-effect of flooring speed. On the forearm stuff I think the motion is bigger, though it shrinks at the higher end of speed.
Biggest compromise on the wrist stuff is that at max speed I’m biting very little of the string, and if I’m not mindful of the attack it can become unacceptably quiet. Lately I’ve dabbled with a variation that uses a setup for the wrist stuff that’s anchored closer to the neck pickup so I can use a much flatter attack with my wrist movement and get a usable sound from 351-shaped celluloid or Tortex picks.
For more moderate speed alternate picking, I use the wrist USX with a bit more arch to the wrist and a bigger range of motion. Depending on the lick, sometimes if I shift into “fast mode” I’ll arch my wrist and do either a fully-blown version of the forearm rotation thing or a forearm rotation thing that’s partway in between. Even the wrist USX feels like shrunk down forearm rotation to me, though I can recognize that the most visible aspect is a wrist movement.
For “walking speed” licks, I tend to do a hoppy DSX movement (but with a downward pickslant) where the attack looks sort of like Mike Stern’s.
You may also have noticed that on the wrist stuff my ring finger is yanking pretty hard against the high E string. I’m not sure tensing and relaxing of that finger might even play a role in making the movement work. When I’m on the high E (and probably on the B?) my hand shifts a little and the ring finger is pressing on the body a few centimeters beyond the high E.
I think my absolute fastest picking is tiny movements with the MAB setup, but if you pulled me out of a frozen lake and said “here’s a guitar, play some Yngwie licks”, I’d probably do the big forearm rotation.
Re: Timing: I wouldn’t be surprised if timing uniformity suffered a bit at the top end. I don’t record myself much, but it’s not bad enough to grab my ear while I’m actually playing.