You will also have reduced control authority (slower stabilization, less power against wind, etc). 50% for pitch makes sense (since there are only 2 motors in the back), but I'd definitely use 100% for roll (my Y6 uses the mix from QGC and flies perfectly). Tune PIDs (especially Tilt Rate D) until MOT_PITCH and MOT_ROLL are well within +/- 100 during steady hover with no wind. Shoot for ~50% motor power required to hover, at least to start with (use smaller props if you're overpowered). But first check ACC values for vibrations.BTW, not to resurrect the old argument, but as I understand it, a shorter arm, while having less leverage, would also need to travel a proportionally shorter distance than a longer arm.
-Max