a downboy 1000 would better than a resistor.
If you have a newer QIII, the step-up regulation board is already ~1000mAh; so running 2 CR123's with a downboy 1000 will only lengthen runtime. Any real improvement in brightness would probably only occur if you ran a very low ohm resistor (hard overdriving the LED at the expense of shorting the life of the LED) .