You need to set both Q_THR_MIN_PWM and Q_THR_MAX_PWM for it to have any effect.
Alternatively, you can set Q_M_SPIN_MAX as a number between 0 and 1. The default value is 0.95, meaning it won’t go above 95% throttle. If you drop that to 0.9 then it won’t go above 90% throttle.
I was wondering, in the case of a tailsitter will Q_M_SPIN_MAX will only limit throttle when in VTOL or will it also work when transitioning into Forward Flight?