To chime in on this (four years later!) I recently picked up a Metabones X-mount 0.71x. I'm posting mainly as I couldn't find much about user experiences with them for X-mount. This thread was pretty much all there is, asides from a guy doing a youtube review but he used a 0.64x which seems odd as there is no 0.64x adapter for X-mount.
I have only had it a couple of days and tried it with my X-T4 and the following EF lenses: Canon 50 1.2L, Canon 50 1.8 STM and Canon 16-35 F4 IS. It worked fine, focus, IS (when available) all worked. I'm not sure how great the focus is, but it did lock on to things. There isn't much vignetting that I could see. Overall it seems fine, I'm surprised more people are not using this adapter with X-mount bodies. The only thing of note is that it is quite tight on the EF mount with L series lenses (which have weather sealing gaskets). The 50mm STM was perfect, the other two took a bit of a twist to get on / off. But nothing extreme. Certainly there won't be any light leaks. I now want to try it out with some vintage glass, though I am a bit worried about hitting the optics with the rear of some of my older lenses. I'm working to the principle that adapted Contax Zeiss lenses that hit the mirror on a 5D will also not be usable on this speed booster. We will see.
