Is it not true that IBIS is most effective on shorter lenses, and that it doesn't do so much for extra long lenses? It might help a bit when added to the lens's IS, but it not a substitute for it.
I love questions where I am genuinely intrigued by what the answer might be. In contrast, I believe that longer focal lengths would get more usage out of IBIS than a shorter once due to the perceived magnification of camera movement at longer lengths. But it really looks more lens based...
Per Canon spec on the applicable lens pages and then here for IBIS...
15-35/2.8 has 5 w/IS, and 7 w/IS+IBIS
50/1.2 has no IS, and 7 w/IBIS
85/1.2 has no IS, and 8 w/IBIS
70-200/2.8 has 5 w/IS, and 7.5 w/IS+IBIS
100-500/4.5-7.1 has 5 stops w/IS, and 6 w/IS+IBIS
So comparing just about any lens to the 100-500 supports your theory... but comparing others such as the 15-35 > 70-200 or the 50 > 85 doesn't. It would be nice to see the data points on other long lenses.
Note: There is also the image circle of each lens in play here too as that matters as well when dealing with IBIS.