Can't raise mot_pwm_max after controller change

This question spawns out of a another tread here Suddenly Copter behaving wildly, altitude drops and overall loss of control

Via logs I realised that arducopter was limiting rc_out to 1960 pwm with my Taranis calibrated to max 2006 rc_3 (thrust).
I’ve changed to a controller who’s rc_3 max is 1890 and now via logs I can see ardupilot is limiting rc_out to 1840.
So I’m losing 120 worth of pwm range/power.
I changed mot_pwm_max to 2000 thinking this would raise the limit on rc_out but it remains at 1840.
I’d like to know why it’s still limited and what I can do to raise this.
I realise that I can probably set the ESC limits to compensate but this would be sacrificing some resolution and is a bit of a work around I’d rather not do.
Is there something I’m missing?

Anyone know the logic behind ardupilots max motor output? Seems to be an overlooked subject. Especially with calibration of your trust stuck determining the output of your motors. That’s a lot of multi rotors our there not getting the most from their motors.