IF you have replaced a piston, then your crank/motor will more than likely be out of balance. Even motors from the factory are usually out of balance, definately hit or miss. Weighing the piston/rod assembly (rotating mass) and matching them to the crank to minimize rotating imbalances is the major way to reduce engine vibrations. Of course that takes money and a builder to do it.
If you have enough vibration that your hands are going numb... that is probably tearing your motor (bearings, seals, mounts, etc.) and frame apart. Bolting stuff on to reduce vibs (bars, mounts, lead, silicone, etc.) is good, in my opinion if you have already balanced the crank. Putting $ into the motor will make the motor, frame, parts mounted to the frame last longer.
I have had the crank balanced/welded when it was ported/rebuilt, pro-tapers, solid bar mount triple top clamp, siliconed the pipe brackets, and rebuilt the pipe mounts. IT still vibrates!! Granted alot less than when I first started, but that was stock. After rebuilding it and you go with more compression.. more vibration.. its a relationship you can't avoid. I have a much stiffer suspension now and that increases vibes as well. So, it is probably back to a medium vibration KX5.
I think the best thing that I have found to help reduce vibration is... Ride in the SAND !! Like a giant sponge to help soak up the vibes...
Sly