Unfortunatly air fuel only accounts for ~7% of power between the point it is so lean that it is starting to break up, to the point it is so rich it is starting to break up. The real power loss comes in well after that. WAY to rich/lean and breaking up you will certinatly loose power, but if it runs well and is safe don't fuss with it. Get the car to run well, and safe and you will be near your best air fuel ratio for max power. Instead, focus on your timing advance.
The head that will give you the best power is going to be the one that has the ports, valves and chambers with the air flow necessary to match your cam, compression ratio and exhaust system.
The head that gives you the best HP at sea level will be the same as the one that gives you the best HP at 5000 feet - whether you are running supercharged or naturally aspirated.