Lets be clear, both Ultegra 6510 & 600 levers are 9 speed compatible. Indeed, they both work with double or triple cranks. However, they differ in the size of the lever. The Ultegra 600 STi levers are for small hands such as those found on women and children.

Hi Chris,
I've just built up a touring frameset with Ultegra 6510 STI levers, an XT front crankset, 105 triple front derailleur and XT 771 medium cage rear derailleur. The 105 front mech is necessary as the throw of the XT front mech is too much for the Ultegra shifters. Remember to put an inline adjuster on the front shifter cable (Jagwire are good).Shifting is superb. Brakes are Avid BB7 Road calipers with compressionless cables and stop very well. Hope this helps.