I have a Gravel X from BBB and a Rebel PW-X hardtail that I bought last summer. The PW-X motor is physically much smaller than the PW or SE. To me they look very different.
The main thing I notice comparing motors is the engagement. The PW-X engages much quickly and smoothly. Aside from...
I think you're likely right re: the SE motor. I remember seeing a publication from Yamaha that pairs the SE motor with the PW-X display. Everything I have seen re: the PW pairs it with the older displays. I spun it up to a bit over 100rpm and didn't hit the limit either.