A Mezz rep posted a while back that the only difference in the WX and WD shaft is the tip. Mezz was exclusively installing Moori tips on their shafts and have now switched to Kamuis. No idea why they would have changed the name of a shaft just because they switched tips.
Also, someone mentioned something about the wavy pin and it fitting another shaft. I have had about 15 Mezz shafts thus far and the wavy joint has been my cue of choice. The only drawback is the joint itself. The pin is bigger than a 3/8 so you cant put any other shaft on the butt. I did find that the wavy pin shafts would fit a few butts I had with a radial pin but not perfect. For a small pin cue the United joint shafts are really nice, they fit nice and snug and hit solid. I have been playing with the WX900 shaft for about 6 months or more and its definitely different than any of the others, not just size wise and taper. The WD700 and WX700 play about the same but both are very different than the WX900, similar to the Predator 314 and Z shafts.
I posted this quite a while back
http://forums.azbilliards.com/showthread.php?t=340336 and have since pulled the wavy pin out of the CJ cue and installed a 3/8x11 and redid the shafts. Its much easier to find a variety of shafts and their is zero difference in the hit.