The voltage drop for 14 ga wire would be approximately 10.6 volts at 15 amp draw.
The voltage drop for 12 ga wire would be approximately 6.7 volts for the same 15 amp draw.
I don't know if this is enough difference to justify rewiring to 12 ga or not. If it were me, and there was a LOT of work involved, I would stay with the 14 ga. Remember, the above voltage drops are only at a full 15 amp draw. Your call.