It seems like the same pixel density. I have used the M5 on my 500mm, other than the flip screen there isn't much reason to use it over the 5DsR.
If I didn't have the 5DsR I might use it to get that little bit of reach.

If it is a M vs extender question, usually just adding one doesn't give you the perfect focal length. It would be a M plus an extender to get the maximum.

I find the M to be a good walk around body to take on vacation. Maybe a good camera to take family gathering pictures. For wildlife or birding it would be just a novelty for me.