If Tamron + adapters is anything to go by, I would say Sigma autofocus would work significantly better on the R system versus a mirrored system, with caveats as usual. On the 5d4, Tamron autofocus was hit and miss for me, and I tried the latest 24-70 f/2.8 G2, 70-200 f/2.8 G2, 35/45 f/1.8 VC's, and more time than I'm willing to admit on microadjustments (both Tamron console and in-camera). Mounted on the R with an adapter, autofocus accuracy noticeably improved, although not as good as adapted EF or RF. For example, I would aim an autofocus point with the Tamron, and it would hunt back and forth, and when I put on an EF lens, it would lock instantly on the same autofocus point, same lighting (in this specific test, I compared a Tamron 24-70 f/2.8 G2 versus a Canon 24-70 f/2.8 II). But overall, I would say on the R, the Tamron autofocus is about 85% of the autofocus speed and accuracy of an RF lens, whereas the adapted EF lenses are about 95% of the RF lens speed and accuracy, at least with the lenses I used and compared (I only have the RF 35 f/1.8 as my sole RF lens at this time).
One area though which was a disappointment with Tamrons (not sure about Sigmas), is video use. Tamron aperture is not "stepless", according to them, resulting in noticeable clicking noises as the aperture changes during video recording. The workaround is to use fixed aperture, but even then, the autofocus noise is also quite noticeable, much louder than any Canon EF lens I have.