Besides increasing WP Nav Speed, who can you do to get your heli to fly closer to it max capable airspeed?
Well, the WPNav Speed sets the speed it will try to fly at. But to do so, you also need to make sure the PID tuning is as tight as it can be. Also the Alt-Hold PIDs need to be good. Otherwise you can command a speed with the WP Speed that will cause the system to lose control and possibly crash.
Also, increasing the WP Accel speed will help you get up to speed faster.