8 speed spacing is 4.8mm whereas 9 speed is tighter at 4.34mm. This correlates to the amount of cable pull the STI shifter will take up at each click. Thus, I'd say nyet.......
http://www.sheldonbrown.com/cribsheet-spacing.html
Between parktool.com and sheldonbrown.com those 2 sites have most of the answers.