Coatings can reduce friction, but will not reduce the main causes of camshaft wear. These are fatigue failure of the surface. The coatings just transmit the forces from the cam follower.
With regards to solid tappets, we have had the best results with super polishing the tappet bottom correctly and with polishing the cam.
It goes without saying that cryogenic processing of the camshaft and lifters will do wonders for wear resistance on solid, hydraulic, and roller lifter cams.