Excuse my ignorance, but if the problem of a larger propeller generating more thrust is that it hits the ground, why not increase the number of blades and thus maintain the original shape of the propeller? It is true that a larger number of blades affects the efficiency but really it might be better than cutting the propellers. A 5-bladed propeller can decrease its diameter quite a bit compared to a 2-bladed equivalent.
The 9x9 for Power Pack C was maybe a tad too large for ground clearance. A 7x9x3 would work well if I could find any. You're right that multiple blades is the way to go for increased performance if diameter is limited.
The diameter for the cut-down props is lower but not due to ground clearance concerns. Because I have to have such a high pitch to get the pitch speed required for high flight speeds, the blade area (i.e. diameter) has to decrease to keep the motor within power limits. Essentially I'm trading off static thrust (less area and stalling the blades at zero airspeed) for dynamic thrust at high speeds.