I have some nice 0.9 degree steppers on hand and tried one on the original direct drive carriage extruder.
Very smooth, but very weak (they generally don't have as much torque as 1.8 degree steppers of equivalent size have).
It had lots of problems even when I turned up the motor current.
So I converted back to the original motor but mounted in a geared Titan/Bowden setup.
I love it.
I'll probably never go back to direct drive on the carriage.
Never say never, but not likely.
To stay on subject for the the thread though, I also considered running a pair of them on my sync'd Z motor setup.
But after the experience on the extruder, it's not happening.
I might opt for 36 tooth pulleys on the lead screws to make an effective 1.8:1 reduction, but that would be some time in the future.