Opticallimits

Awesome lens .... https://opticallimits.com/canon/canon-rf/canon-rf-10-20mm-f-4-l-is-stm-review/

Yeah, this looks like a lens to choose the system for... if you have deep pockets. Though the "me wanna" factor is mitigated by the existence of the Laowa 10mm - guess it will have to suffice for those in the other systems, like myself.
ZOMG @ 9% distortion and 12 stops of vignetting in an L lens though... but last I heard, the 14-35 is like this too, though maybe not that drastic (my uneducated guess is around 6% distortion... see how close I am when you review that lens. )

12 EV down to 2 EV? I suppose the extreme corners are mostly noise... or not?

With the distortion correction, they're just cut off, mostly.