Originally Posted by
stephtu
My understanding is that if going for MTB derailleur for more tooth capacity (probably good to have even if you switch to smaller cassette, in case you ever want to switch back) in fact it *has* to be 9 speed, not 10 speed. Shimano MTB 10-speed having different amount of cable pull vs. derailleur movement.
So with Shimano road STI, use any road derailleur, or 9 speed MTB "top-normal".
9 speed MTB RD works fine with 10 speed drive trains. On the SRAM side, people have used the X9 RD (medium cage) with the 11-36 cassette and a 50/34 compact to get different gearing.