If you want to reach sharpness nirvana, you might want to try a shapton 30K glass plate. If I remember correctly, some here have described them as giving feather sharp edges.

Personally, I am very happy with my 8x3 coticule and the edge it gives me after the norton 4000/8000.

I suspect that also has to do with the fact that I like wide stones, and while a 3" wide coticule is affordable, I would break the bank trying to buy a 3" wide escher (I don't even think they exist).
3" wide Japanese high grit stones cost a fortune, so for now I am content with my coticule edges.

If and when I am able to hone every razor to the max potential of the coticule, every time, then I might invest in a finer stone.
Until then I have more important needs, like a good belt sander, a drill press and a vise.