that is really interesting - mine was fitted with the 375g weights with attached handguards and original mirrors. But there were signs that previous owners had still suffered with the vibration
(1) bits of sheet rubber between the bars and the clamps
(2) many many ball bearings inside the bar end secured with araldite
So i put a lot of time into getting the valve clearances exactly the same between cylinders and synchronising the throttles as that used to tame the vibration of most carburetted ancient twins and multi-cylinder bikes.
But it still "thundered" through the bars at high speed so as an experiment i took out the ball-bearings and the rubber strips, removed the weights and also the mirrors and put cheepo Fleabay mirrors on the bar end.
Over the last 200 mile it has been better.
I will try it with better & narrower mirrors but as the mirror and handlebar arrangement is rather like a tuning fork I would not be surprised if that amplified some vibration frequencies