1st things first, I dont care what octain you put in a car you are loosing power and the car is not going to perform correctly until he fixes the exhaust leaks, I would also assume he has several vacuum leaks, once those are fixed I would set the TPS by using a volt meter or investing in a scanmaster, At that point changing the plugs to a stock replacement set of ac delco or autolight plugs gapped at about .024 and drive the car with some good fuel. At that point posting your findings would probably result in a better picture for us on the intraweb. Until those things are fixed you are 100% pissing in the wind.