well the American speedway racers say its good, they also reccomend "water wetter". no too sure about the mix ratios though, seems a bit funny to me.
"Keyser Manufacturing 40 Below coolant additive is designed to reduce coolant temperatures by up to 40 degrees F. It can stop your radiator from overheating and is ideal for racing or other high performance engines. The additive maintains coolant temperature between 180-210 degrees F when one 30 oz. can is added to a mixture of 80 percent distilled water/20 percent ethylene glycol (antifreeze)."
"Reduces coolant temps up to 40° F. Prevents coolant temperature from rising excessively under extreme racing conditions. 30 oz. Use a 80% water and 20% Pro-Blend 40 Below mix.
NOTE: Do NOT use with propylene glycol antifreeze, stop leak, or block sealer. Can be used with distilled water and ethylene glycol antifreeze."