I hadn't considered that it might drive a lens faster. Interesting. Ultimately, volts are volts, but I wonder if there are momentary drops in voltage with one battery that is fixed by having two? Or maybe as the battery drains it's performance lessons, despite still being within an "acceptable" spec. With two batteries, that drain takes longer to appear? I am seeing something like this with my M6II. I haven't covered this in a post yet, but there were a couple sequence of images where it just did not get focused even though the scene had good contrast. I did not notice until I was at home looking through photos, but in retrospect, the 2-3 events occurred when the battery was low.
BTW, I should probably clarify, my 5DIV would be ~400-1,200 images, per battery (mix of batteries from 2013-2017). So the higher end is inline with what I've seen on the R5. My main issue with batteries and the 5DIV is actually drain while the camera is off or powered down. A fully charged battery will be dead in a week or so. If I mistakenly just pick up the camera and go, the battery might be dead. My fix is that I have multiple fully charged batteries and charger sitting on a shelf for easy access. Whenever I grab the 5DIV, if I haven't used it in a day or two, I swap out the batteries. But, I have yet to see any appreciable drain on the R5.
But I agree, I am also enjoying the compactness of the R5. This gets a bit more complicated as I shoot enough off a tripod that the "L" bracket is important and, of course, there are different "L" brackets for gripped vs non-gripped configurations.
Also, and I do not know if you use a "L" bracket or leave it on, but I was just looking at the RRS images/videos. While it looks like RRS has worked to accommodate the swivel screen, but that is a bit too tight of fit for the screen and L bracket for my comfort. Plus...more size/weight.