The manual says to sync carbs at 1750 RPM.  I've heard that even when sync'd at 1750rpm, the balance goes off as you increase RPMs.   Would there be any advantage to just syncing carbs at 5k RPM?

Well high rpm's the engine takes control ... smaller revs are the better place to do it ... say one cylinder seals better than the other (very common) it will never balance up high ... and if you do balance it up high it will be so far off down low ... If your motor is perfectly blue printed and balanced then you can do it high or low and it will be the same ... if not ... the high rpm balance may leave it not even running down low ... low rpm balance will at worst be a bit extra vibes at higher rpm ... and for that look at this thread ...
http://www.gstwins.com/forum/viewtopic.php?t=19486
:lol:
But seriously - high rpm is motor ... carbs are sorta secondary ... vibes and shakes all are from the motor ...
Cool.
Srinath.