I guess the usefulness of high speed video depends on the individual. He did say it was for coaching.
There is no 30 second limitation with the Casio cameras, so High speed video length is determined by the size of the memory card. Chas Tennis will be along promptly, but I can give my 2 cents. The ZR1000 will probably serve you well, provided you take the video outdoors. The thing about the newer Casio models is that they removed the option for manual exposure control, so the camera will choose the shutter speed for you. If you take video indoors for example, it may choose a slower shutter speed and thus have more blurring of the ball and racquet head. So far the only models confirmed to have manual exposure control for HS video are the ex-f1, fh100, fh20, fh25.