Well, you've answered your own question. 
Even the 50L RF does, as you say, "vignette like crazy", despite being used on an allegedly better mount. I've checked a few reviews of Canon mount ultrafast (f/1.4 or brighter) lenses here on PZ and, as a rule, the vignetting only gets better by 85mm (1.8 stops on average for 85mm f/1.4 lenses wide open, same for the 1.2 L). At 35mm, the Sigma f/1.4 and the original Canon L are, interestingly, a little better behaved in this regard than the L II (2.65 and 2.4 stops WO, respectively).
Even the 50L RF does, as you say, "vignette like crazy", despite being used on an allegedly better mount. I've checked a few reviews of Canon mount ultrafast (f/1.4 or brighter) lenses here on PZ and, as a rule, the vignetting only gets better by 85mm (1.8 stops on average for 85mm f/1.4 lenses wide open, same for the 1.2 L). At 35mm, the Sigma f/1.4 and the original Canon L are, interestingly, a little better behaved in this regard than the L II (2.65 and 2.4 stops WO, respectively).