It depends. In theory, the 11x6 will give you more striaght line speed, but less climb than an 11x5. The 11x5 should give you a shorter takeoff roll and better climb.

However, a lot depends on the torque and RPM abilities of the engine. It's possible that the 11x5 could give you better climb and a higher straight line speed at the same time, since it would load the engine less, and you'd have a higher RPM. This might or might not be a good thing for you. On the other hand, if the 11x5 is too light of a load on the engine, you will produce a lot of noise and heat and not much else.

The engine manufacturers are usually very conservitive with their prop ranges, you're far better off trying it and seeing for yourself. You won't hurt the .46FX with an 11x5.

I'd say an 11x5 on a .46 FX would be fine. A 12x4 would work too.