The resistor won't increase the current in the field, so the only thing the resistor would do is get VERY hot and increase the armature current. However, that would reduce the already low current in the field, further impacting performance.
What finer windings would really do for a universal motor is to allow it to run on higher voltages. However, the armature and field should be matched for the best results. I'm 99% sure that a significant mismatch in the armature and field windings will result in a lower performance motor overall.
