Not my area of expertise but you could limit the throttle high point to limit the voltage the motors see- however I would risk it as its not a huge increase and 4  fully charged nicds would exceed this so the motors must have a tolerance to accomodate this.

Are you sure 1w x3 is what you need? That is tiny! Surely not?! 100w per lb is more realistic unless you have crazy low wing loading?

All the best