Detail Man is right! You could gain 3-4% by using a 10.5" prop. In theory, the slipstream contracts after passing through the prop, so your idea might work. If the vehicle is close to the ground, the increase in slipstream velocity caused by the contriction may help the ground effect, but that's just a guess. However, I suspect the real problem is the propeller itself. VERY few propellers are optimized for static conditions. I don't imagine that the GWS props are particularly efficient anyway. The best propeller will NOT have a constant pitch distribution along the blade. It will have an increasing pitch towards the tip, making for a flatter twist. You may be better off to try and make your own, like a helicopter blade, maybe tapered so the tip is 1/2 the root or something.

I don't suppose you know the torque or power of the motor? Current draw?