The BEMF adjustments are limited on the Sapphire, but CV 150 has 2 settings to suit different motor types. CV 150 can be set to either 0 or 1 and if I remember correctly a value of 1 is for older motors. Might be worth a try....
But you have to bear in mind that this motor will never run as well as the MM locos. MM's are equipped with a good quality can motor and flywheels which improve slow running considerably, you will probably get the 58 motor running smoothly but it's slowest (reliable) speed will be higher than an MM. I would concentrate on getting the loco to run steadily at speed step 1.