Shimano "B" Cable Position For Rear Derailleur
 

I want to change my STI shifters from 9spd Ultegra to 10spd (because the ergonomics are better), but I'm not sure I want to go ahead with a complete conversion to 10spd including cassette, chain, then possibly front rings if the system is worn, etc.

Shimano systems can use an alternate "B" position for the derailleur cable which changes the cable-gain and allows "N" shifters to be used with "N-1" drivetrains. Therefore 10spd shifters can be used with 9spd drivetrain, 9spd shifters w/ 8spd drivetrain, etc.

I was thinking of trying this. Does this actually work? Is the shift quality good? Or am I going to wind up completely pissed off?

I've tried that on a 9sp shifter + 8sp cassette.
It worked and shifted perfectly.

However, the set-up was a bit tricky.
Since, there is no cable-groove cut on the RD for the
alternate position it took me a couple of tries to get the
cable onto the perfect position.

Tip: Once you nail down the cable position,
mark the spot and cut a groove with a
file or a hacksaw to take the hassle
out of future cable changes.

Why bother.

You can get the adpater that changes the pull *perfectly* without any hack modifications or alternate clamping positions that yield **** performance for $40. Go buy it.

http://jtekengineering.com/shiftmate.htm

+1 on the J-tek shiftmate. I used one to run a 9-speed Shimano cassette and Shimano rd with 10-speed Campy Ergos and am currently using one with a 10-speed Shimano cassette and Shimano rd with the same Campy Ergo brifters. The shifting is perfect.

BTW, since brifters are by far the most expensive single item in any upgrade, why not just go for a 10-speed cassette and chain too. If your chainrings need replacing, they need replacing. The change to 10-speed won't have any effect on that either way.