The biggest limitation on the m103 engine is the design of the air box and intake ducts themselves. If you look at it you might think you are getting a ram effect with the origin of the intake just behind the grill next to the radiator and condenser. Well good racing design removes all bends, heat and whatever so your cooler denser outside air get a straigt shot at the induction hardware. You have a stock air box that's huge with a lot of room for turbulence and redistribution of airflow with a narrow intake duct that changes shape at least two times. An air filter might clean this up a bit, but you're still stuck with an air handling system designed for average use. Can't make a silk purse out of a cow's ear, but you can have a lot of funn trying.
1987 BMW 325