First of all, tire growth at high speeds will definately affect it and the only thing you can do is add a little to the height of the tire when making your calculations. You will never be perfect for all speeds (unless you go to GPS), but you can make your average speed more accurate.
As for using the resistor plugs, they certainly would help if it were an interference problem. The resistor is put in the plugs to keep your plugs from messing up other electronics. Have you ever had an ATV or snowmobile drive by your house and screw up you TV or radio? That from plugs without the resistor.