I would suggest you have your parts worked by Liberty Gears.
Cost about $350. I've tried shifters, magic lubes and they help a little but the street speed upgrade by Liberty made my 600 shift like a T-5, smooth and fast.
WELCOME TO LIBERTYS HIGH PERFORMANCE
We torn down my 600 and shipped the required parts to Liberty. A couple weeks later we install the reworked / new parts. Took about two hours to take the trannie apart and about the same two hrs to put it back together.
It's one of those things I should have done years ago!
You can not miss a gear with this modification!
Dwight