Overall no. Just an upgrade in terms of battery & display, maybe even camera, but otherwise it will be a downgrade in terms of build, performance, experience (full screen display is great), etc.
M51 isn't out yet, no idea when M51s will come but both are like upper mid-ranger vs flagship killer Asus 6z. Let's say M51s gets S735 (assumed successor of S732), that will still be nowhere near S855 in performance. For reference, my 3 year old S8's S835 matches S765 in CPU power (similar), but S835 still holds a lead in GPU side, efficiency is surely better for S765 as its newer.