MSD Ignition Installed

I've always used MSD ignitions, and have never used a resistor. Try putting a jumper across the resistor and see if it still runs okay. If it does then its something else in the wiring. Minimum voltage for the MSD is 12v, and at 14v it runs better. When you run the resistor it drops the voltage, so in "theory" you're holding it back.
But whatever the case, I get the same reaction from people whenever I've installed a MSD into their cars. I love those things :D