Yes, this is a question that has been coming up for years. Now, in 2020, Canon has given us two full-frame IBIS models that work with IS and non IS lenses.
I need to do some controlled tests with subjects moving at moderate speeds, but, on the R6, I'm seeing modest but surprising motion blur in images up to 1/1250th of a second. Some where they are walking, some where they are jogging, and some where they are standing still.
I don't think this discussion must be limited to R5/R6 users. What is your IS/IBIS strategy for handheld shots? Do you turn it off above a certain shutter speed? For all subjects? Moving? Still?
What if your subjects are going past you, but your lens doesn't have a Mode 2, or panning IS?
I did some tests today using a Canon R5, Canon 100-400mm 1:4.5-5.6 L IS II USM, and a Canon 1.4x Extender EF III mounted on a Benro tripod.
The camera and lenses were about 50 feet from a flat target point with a slanted target beside it, much like a Squit Photo calibration, in full sun.
I was trying to test the following: IS or NoIS. Servo or Single Shot, Full Frame versus Crop, Teleconverter or No tele with crop in post, Focusing with magnification or straight autofocus in camera or liveview, and shutter speed. Lots of combinations but I'll summarize.
The easy one is shutter speed. 1/200 at 400 and 560mm was soft but acceptable. Higher shutter speeds had sharper results. 1/2000 worse than 1/4000, worse than 1/8000. Faster was better, no new knowledge there.
Full Frame was better than crop in camera. Both were sharp but the full frame with a crop in post seemed like it had more detail and contrast. I will not be using crop but will just crop in post. My focusing didn't seem to be any better using the crop feature. Maybe my technique needs refined.
IS was always sharper than non-IS in every scenario I tested. With and without the teleconverter.
Using the Teleconverter resulted in a bit softer images however I didn't adjust the position of my camera by moving the 400mm tripod position to equal the 560mm tripod position, nor did I adjust the f stop accordingly. I would still use the T.C. in the field under the right conditions unless I don't need it, then native is better.
Single shot was sharper than Servo. I think that was the biggest takeaway for me. When shooting a static target from a tripod I'll be turning off my Servo mode. I tend to leave it on all the time, no more in that shooting scenario. Live view,. and magnified focusing were both soft.
I believe the biggest variable in the magnified focusing and live view was the camera shake I introduced by touching the camera while it was mounted on a tripod. Those ever so slight touches cause a soft image. I should have tested with a remote release to isolate this variable.
Those were my tests as I was trying to learn how my new R5 operates and is matched to lenses and conditions.
Hope this helps. I'm open to others doing the same type of tests.