Are you sure that Sigma AF for DLSRs are as good as Canon native EF lenses released relatively recently??
Let me just say "it's good enough to not feeling any limitation or difference whatsoever".
Explanation: on 6D i had Sigma 50 & 135 Art and the Tamron 24-70 2.8 VC G1.
On R6 I switched the Tamron (all G1 lenses AF is known as being sketchy on Canon R, and certainly mine was; only after selling I discovered that Tamron service could flash the firmware) for the Sigma 24-105 Art, and later I got a very lucky shot in the used market and switched the Sigma 24-105 for the Canon EF 24-70 2.8 L II, which is also the most recent EF lens I purchased and used extensively, and so it's my term of comparison for "how EF stuff should behave".
Then, later I sold the Sigma 50 and 135 Art and bought Sigma 40 & 85 Art.
So I had a decent amount of use of Sigma recent primes on both DSLR and ML, and use of recent Tamron zoom on DSLR and Sigma zoom on ML; and of course extensive use of recent (24-70 II) and less recent (70-200 non-IS) EF lenses on both DSLR and ML, and use of TON of stuff I had and used over the last 24 years, both Canon (24 STM, 35 f2 40 STM, 50 1.4, 50 1.8, 85 1.8, 100 f2, 135 f2, etc etc etc) and third party (Tokina 11-16, 16-28, various old Sigma non-Art)
From my personal experience,
and I mean "personal", so let's not get to absolutes because I've no interest in getting into cat fights, I can surely say that, excluding the Tamron 24-70 on ML,
any recent third party lens I used (which, excluding that Tamron on ML, basically means just Sigma Art and the Tokina's on DSLR) showed no difference of AF precision, speed and general reliability (lost contacts between camera and lens, IS issues, mech issues, etc) compared to my reference EF 24-70 II.
In a blind test,
in my experience, and for what I do and how I use camera and lenses, I can't tell apart the EF 24-70 II from any Sigma art lens I owned, and can't tell the Canon apart from that Tamron when used on DSLR.
As I can't tell apart any recent (Canon or third party) EF lens from any RF lens (16, 35, 50 and 85, all STM) I owned and used in the last 2 years. EDIT: no, wait; actually I can tell apart the 85 STM AF that was really REALLY slow and hunting, especially when backlight, that's why I resold it at the speed of light.
Of course YMMV, but frankly of ALL above mentioned, the only thing I bought new is the 40 Art (and bought it online), all the rest comes used from eBay, so I wasn't going to the shop and asking to test 3 or 4 Sigma copies and come home with the golden lens. So can't believe I'm so lucky I got all perfect lenses, and all the rest of the world got terrible lenses plagued with AF inconsistency.
Then, I do studio & location portraits, and weddings; not doing BIF's, not doing sport, etc, and I shoot at 1fps; if there's a difference in "extreme" conditions, meaning super fast subjects and/or super high framerates, where third party lenses loses their grip, that's another story, and I don't discuss that there can be a difference there.
But for what I do, no, any Sigma Art I owned is or was as good as my 24-70 L II.