Increasing or decreasing cable tension in small increments with the adjuster can make a big difference in fine tuning RD operation.
http://www.parktool.com/repair/readhowto.asp?id=64
I'm used to the adjusting barrels on the SRAM grip- or triggershifters that I use. Not familiar with STI.
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Bacchetta Giro A20, RANS V-Rex, RANS Screamer