I got the same problem and initially, I change the 50% to 80% to allow more power (changing parameter here https://github.com/ArduPilot/ardupilot/blob/master/APMrover2/Steering.cpp#L287)
Then I try to make this 0.5 as a parameter https://github.com/khancyr/ardupilot/commit/c8c1290aba408b8821022fa5ec430ae2d3930286 .
But I don’t think it is the right think to do. I reviewed my rover parmeter (specially CRUISE_SPEED CRUISE_THROTTLE etc) and I can reach nearly 100% turn speed (according to log pwm output)