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".