Well, we haven't seen Yamato do any 100% serious fighting, he's mainly been there to support characters. Honestly I think it'd be a good fight, given Yamato doesn't get caught in a Genjutsu.
Look at it this way, Kakashi's got a Sharingan so he can predict moves, however Kakashi doesn't really use anything but Lightning Cutter, and that hasn't improved much since the Kakashi Gaiden. He's got the Sharingan so he can predict the movements and actually hit with the Lighnting Cutter, but if Yamato's quick enough all he's got to do is play a pure defensive role against Kakashi and use Great Forest Technique whilst Kakashi starts to run at him.
-And then well, lets just say Kakashi would most likely be skewered & ready to roast.
Sharingan allows you to see movements and predict movements, but that doesn't mean the person can actually react quick enough to have those predictions matter.
It's really all about who gets early game upper hand, and situational things like how far they are away from each other ect.
As soon as Yamato's caught in Genjutsu it's over. =P