As mentioned on this thread, the 15.5% (500 hp) is probably the maximum that could ever be expected at the tail-rotor. However, I did some calculations based on this and the data on Wikipedia.
Eliminating the tail-rotor appears to show a substantial increase in the payload carrying ability.
During this maximum yaw the Maximum Power going to the main rotor is 3,240 - 500 = 2,740 hp.
The Maximum Takeoff Weight is 21,884 lb.
Therefore, the lift capability is 21,884 / 2740 = 8 lb/hp.
This implies that if the 500 hp going to the tail-rotor were sent to a second main-rotor, which was aerodynamically remote from the first main-rotor, the craft would be able to increase its payload by up to a theoretical 500 hp * 8.0 lb/hp = 4,000 lbs.
The Empty Weight is 13,648 lb., which gives a Maximum Payload is 8,236 lbs.
The additional 4,000 lbs of payload represents a hypothetical maximum increase in the carrying capacity of this helicopter of 50%, during maximum yaw or sideward flight.
Dave