Other than going to 16.00, I can't imagine a tire that would give more torque against the brake than the 14.00 R20. It's going about 28" further per revolution than a 9.00-20 which is about 20-25% greater torque than the 9.00. I can't imagine a better reason to use larger wheel cylinder pistons (and re-new all of the other components while at it).
Just for comparison, the area of a 1 3/8 piston is 1.512 sq in. The area of 1 1/2 piston is 1.767 sq inch. At 500 psi the force pushing out on the brake is 756 lbs on the 1 3/8 and 833 pounds on the 1 1/2" piston. At 1700 psi, the force is 2570# and with larger piston is 3003# (so closer to 20%).