I own a 7D2 but rarely use it for BIF as it has a dated AF and poor ISO performance. But I would choose the 7d2 over my 5DSR for BIF in some circumstances because it has advantages such as superior FR and buffer.
I think it's fair to say you want a higher frame rate or deeper buffer for action shooting. But it is personal preference, and others will make the trade off for the exceptional resolution of the 5Ds/sr (or other high MP body in another line).
Shutter lag exists with all cameras but is an important consideration when shooting in AI-servo. Cameras with more shutter lag make it more difficult for the camera to accurately predict focus when shooting fast moving subjects.
Pre-focused shutter lag is a measure of how fast the camera can move the mirror out of the way and fire. Shutter lag with AF is a measure of how fast the camera can acquire the target (determine focus and drive the lens to that position) and move the mirror out of the way. Shutter lag with AF is obviously affected by the lens itself and by the amount by which the lens is out of focus.
You're correct that bodies like the 1DX2 can drive the lens faster during initial acquisition for a shorter AF shutter lag, but other Canon bodies including the 5Ds/sr can drive it pretty fast as well. As for pre-focused shutter lag, I believe that's nearly identical among Canon's "pro" bodies even going back a generation or two.
Once a target is acquired tracking AF movements are relatively small. There's no voltage related advantage here and the shutter lag is essentially the pre-focused time
unless the camera has lost the target and is trying to re-acquire.
It does not drive the AF servos of super tele lenses as quickly as the 5D4 or 1DX2. It does not handle tele-converters well and lacks the cross type AF points of the 5D4 or 1DX2.
I think you're making a mountain out of a small hill. There were certainly improvements in the 1DX2 and 5D4. But they were refinements to an AF system that has been refined for three decades. The AF system in the 5Ds/sr, which you're suggesting might cost you a critical moment shot, is equal to or better than the AF systems in all of the EOS 1 and 1D bodies prior to the 1DX2. I don't recall 1D3 owners complaining about missed BiF shots due to slow AF. For that matter I don't recall EOS 1v or EOS 3 owners complaining about this either.
I don't have the stick time on a 5D4 to truly judge the improvements. I believe Alan does and would trust his input. But again, if you want high MP shots of BiF the 5Ds/sr are up to the task. If you want a faster frame rate and deeper buffer there are other choices.