It's about 2.6 in 4k and drops down to about 1.8 with the speedbooster. I have one of those custom ones and it's pretty dang neat-o. I wouldn't use the 4k unless I were shooting manual (focus peaking ftw) and either moving slowly or not at all. Lots of rolling shutter. That being said, I love the way it looks, but I can't do much with it unless I'm doing an interview or just some b-roll.
2.6! That's interesting. Andrew Reid, in his article on the M50 as a Digital Bolex says the crop is 2.34x while Dave Altizer said it was around about 2.45x.
Yes, the use of the 4k is limited mainly to interviews or slow pans.
I wonder if someone, like the engineer in South Korea who made the EF-M EF speedbooster, would like to also create an EF-R to EF one with a 0.71x reducer. That would reduce the 1.8x crop to something more usable (albeit you would have no access to the EF-R lenses).