I did notice that when I brought the files into Topaz DeNoise that it always wanted to use less denoise settings than files I would import from the 90D. The 90D files averaged a denoise setting of between 8-11 on average, whereas the 5DS would range from 1-4. And I use very similar settings between the cameras, the only thing I might change is the aperture.
With the 80D I don't think that Topaz Denoise AI on auto has gone above 4 (and that's rare). It makes sense that the 90D would be noisier considering the increase in pixel density.
I have an 5D on order, I pre-ordered it on launch day but apparently not quite early enough as it still hasn't shipped. I'm curious to see how it works with the MP-e65 lens, and if the IBIS provides a little more flexibility when using the extra magnification
If you're using a flash I doubt that any form of IS is going to be beneficial. Curious to know if it helps when shooting with natural light around 1x. Would also like to know your impression of focusing with the EVF and the MP-E. I shoot a lot of active subjects and I'm not sure if the current tech is responsive enough for tracking a critter in motion at 2x and higher mag.