Look at the Blue Bird BLS-29A. I'm running mine on 6V for steering and it is lightning fast and strong. I was skeptical of the specs for $70 but it's fantastic so far. I've got an ongoing review thread to track how it holds up over time. 330 oz in and 0.068s at 6V. A lot more at 8.4V.

The BLS-36A and BLS-42A are a little slower with even more torque.