"Now I'm not sure I want to get rid of it. Especially because of the MFD difference compared to the 800 F11."
I have both the 150-600 C and 800 f11. I bought the latter for (1) weight considerations and (2) the ability to see more of my subject in the view finder. I, too, was concerned about MFD, as well as the f11 requiring light. In practice, neither of those concerns have proved very limiting.
I trust the 800 enough to use it to capture mega vagrants to the East Coast. For transparency sake, I did have a Pacific-slope Flycatcher land about 6' away from me, which momentarily had me panicking and backtracking for distance. But the little cutie bounced to about 20' and the opportunity was saved. Generally speaking, getting within 18' of little ones is tough.
The f11 light factor has not been much of a problem, at least in my opinion. The R6 handles noise extremely well; couple that with Topaz DeNoise and you're in good shape. Shown is a Screech Owl. It was shot handheld in deep woods (thankfully no leaves) looking up approximately 100' into a hole that looked black from the ground. You can't see feather detail, but I came away satisfied given the conditions.
Something to bear in mind is that the 800 is not going to be a lens of choice for BIFs. It just doesn't work well for that. The 150-600 gives much better results for BIFs. So, if budget considerstions allow, having both is worthwhile.
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Photo from steveb108's gallery.