The problem I see with plug reading is that the plug can see a variation in operating conditions throughout the rpm range, especially on street cars that are drag raced. How long [ in seconds ] does it take to 'colour' a plug? What good is it if the colour is 'right' after a WOT run, but the plug fouls [ & loses time ] at idle & lower rpms because it is too cold. I believe too many people run colder plugs than the engine wants/needs. Wrap some alum foil into a shape of a cigarette & hold a lighter under it. Gets too hot to hold very quickly! Now hold the lighter under a 650 lb cyl block & heads. You will be waiting for ever for it to heat up.......
My point is: how quickly can the plug heat up during a 11-12 sec 1/4 mile run with 650 lb hunk of metal also absorbing the heat. I would stick with the heat range that the manufacturer designed & tested for. I can think of only a few instances where a colder plug would be needed. A hotter plug might be needed with a worn engine.