Good question.
I'd predict no at 20fps and yes at 12fps.
It's helpful to look at what the 1DXiii is capable of video-wise as this is probably the bigger bottleneck in terms of card data writing:
5.5k 12-bit raw @ 60fps. The frame is 5472*2286 -> 1073MB/s
Canon hasn't confirmed the bit depth of the R5 raw video - we just know it's 8kDCI (8192x4320) and 29.97fps. Let's look at how many pixels/second each camera is pushing in raw video at their highest frame rates:
1DXiii: 750,539,520 pixels/s
R5: 1,060,621,516.8 pixels/s
So, the R5 is pushing 41% more pixels in raw video over the same CFexpress interface at its peak frame rate compared to the 1DXiii at its peak frame rate, so I'd guess Canon will be nerfing the bit depth a bit compared to the 1DXiii. At 8bits, the data rates at the highest frame rates are very close (So, I'd predict the R5 has 8 bit raw video, at least at it's highest frame rate. At 23.98 fps it could conceivably fit in the 1GB/s envelope at 10 bit although I kind of doubt that Canon would go to that sort of trouble)
Now onto images:
raw image files will be a full 8192x5262 (3:2) pixels.
If those are 14bit, 20fps yields a data rate of 1494 MB/s - a bit higher than the data rates we see in the 1DXiii and enough so that I doubt the CFexpress interface could keep up (especially not with cards that are available today). At 12fps, the data rate drops down to 896 MB/s which is in the envelope of what the 1DXiii can manage.
So with mechanical shutter you're probably never going to fill the buffer. With electronic shutter, you'll start off with a 20fps burst and then drop to 14 fps or so once the buffer is saturated - which is still pretty respectable.
If you want to shoot this way, you should probably be saving up for CFexpress cards as much as you're saving up for the camera.
Also - this totally ignores the UHS-II SD card which is much slower.
