Don't forget the Yami's had a primary ratio change in 1989. The early bikes had a 31/74 primary and the later bikes had 34/71 ( no that's not a misprint!).just having a search on info on the older XT 600 and it seems the gearing was 15/39 opposed to 15/45 mine is on which could explain the better speed.
This change, along with the different final drive ratio meant the gearing was about the same.
For the gear heads...take a look at this website.
http://www.gearingcommander.com/
