I got the idea from a guy named Gangly Jeff. His website (http://www.geocities.com/MotorCity/Factory/1935/TrannyMod.htm) suggests pounding lead inside the roller weights inside the variator (CVT) to make them heavier and change the acceleration characteristics.
The trick I found that made an even bigger difference, though, is to cut notches in the side of the variator housing to allow the weights to roll an extra half inch or so further. This allows the two sheaves of the pulley to push even closer together and increase your gearing a bit more.
With this mod my PA-50 sounds like it's shifting gears. It'll get up to a certain speed (about 40km/h (25MPH)) and then inertia will push the roller weights into the "extra" space and the engine RPMs will drop as the gear ratio changes pushing the engine back into the power band. I'll get a power boost good for about 10-15km/h (6 - 9MPH) more.
I'm not at home this weekend, so I can't take photos of the modification. I will take some pictures and post them after the weekend. It's one of those things that can best be described with a couple of pictures rather than with ten pages of text.
In the meantime, I recommend lubricating the back, spring-loaded belt pulley. You might find that making sure that it slides properly open and closed will give you a bit more speed (don't lubricate the pulley faces though!).