I'm using 10-speed Dura Ace bar end shift levers (the same SL-BS79) mounted on Gevenalle "brifter" brackets to shift both a 9-speed Ultegra RD-6503 rear derailleur and an 8-speed 105 RD-1050 rear derailleur using Shimano 10-speed cassettes. Both set-ups work extremely well.
Shimano road/MTB shifter/derailleur compatibility was great from 7-speed 105/Ultegra/MTB groups and 9-speed Dura Ace right up until 10-speed MTB and 11-speed everything.