It has everything to do with torque. Think of it as a wrench. If you have a longer wrench, it doesn't take as much force on the end of it to turn a bolt. If the wrench is shorter, you have to put greater force on it to turn the bolt. Same thing in the plane. If you have a longer arm (ie. distance from CG to the rudder), than you don't need as much force acting on the rudder to keep the plane from yawing. Since the rudder is fixed, the only way you can make the distance longer is to have a forward CG. Therefore, if the CG is moved back (bad for Vmc), then you have to fly faster to get the same amount of force.

Performance, however, is different. The reason it's better for performance is the same reason as it is with two engines. If the CG is moved aft, you no longer need as much tail down force. This gives less "induced" weight. Less weight = better performance.

Make sense, or do I need to work on my analogies?