The thing I really like about Heath Waters's work is how he's incorporated some biomechanical concepts into tennis instruction. Stuff like the myotatic stretch reflex and the stretch-shorten cycle into tennis. The SSC is pretty powerful; it can increase the effective "power" of a muscle up to 100%, if only for milleseconds. This is pretty much the basis of plyometric training; basketball and football players are well acquainted with this.
That said, I do kind disagree with his interpretation of the SSC for the modern FH strokes here and there. But it's good to know that this kind of information is getting out there.
On other boards I travel, this and other concepts (i.e. prioreceptors, neural drive, rate coding, GTO inhibition, irradiation techniques, etc.) becomes part of the discourse when breaking down any performance movement. It's a very, very important part for powerlifting and OL competitions, for example. It gives you another way to look at the problem.
That said, I don't know how much more value is in subscribing to another site if you're already hooked up to tennisplayer or hi-tech. TP has maybe the best pure instruction; and hi-tech has some of the best, most useful camerawork.