An 11 X 4 might be a little underpropped on the FX, but I've never had one so not sure.

A 10 X 6 however, will definitely give you more speed, which is what you say you don't want. If you have the ground clearance, I'd suggest trying a 12 X 4.

Not sure why pulling back the throttle isn't sufficient, unless you're looking for a "braking" affect from the prop.
Dennis-