Changed out my Ultegra STi levers for DuraAce this weekend, and changed gearing as well. Went from a 54 Lg chainring to 52 (42/30 on the rest of the triple crankset),and from a 12-27 (wore it out...) to 12-23 cassette (had it in the parts bin). Was a bit skeptical about putting the 12/23 on, but since I never really used the 30 and was seldom using the 42, this has resulted in a very smooth shifting system that gives me more useful gears, since I now MUST use the 42!

The DuraAce levers make a big difference as well: quicker shifting and better braking power, believe it or not.