The guy has definitely built some blades people seem to like. He is willing to do what seem to me to be some pretty unconventional things with wood, also. He was insightful enough to start marketing seamless balls early on (even though he doesn't make those himself, and he made some fairly inflated claims about how his are somehow different and better, which was not even remotely obvious to me, and I feel like I know quite a bit about balls).
Compared to everything else in TT equipment design, rubber is a whole different thing, and specifically requires are pretty high level of technical expertise in a LOT of different things, which makes it orders of magnitude more complicated than almost everything else in the sport. Thing is, I can't say anything about stuff I haven't tried and I have never tried Karis. This guy's diaries at MyTT get my BS detectors vibrating, sort of like a special Spidey Sense. Of course, unlike Spiderman, my version is not infallible. Even though I am pretty skeptical, I would definitely be interested in hitting with it in the unlikely event someone here shows up with a sheet.
Like you I am sure he discusses his stuff with good players but my experience is that good players have really good hands for feeling when they like something, but in a lot of cases would not be able to tell you anything about what went into the design of the blade or rubber that caused them to like it. (I am also bad in that way, threads where people talk about how wood affects the playing properties of blades, for example pass me by).