I think 50% is the theoretical maximum efficiency, I recall there were some calculations to demonstrate this. The heat transfer coils were usually 4 or 6 row, due to the small dT between the water and air. The theoretical heat recovery didn't make any allowance for the load imposed on the fans by the coils or for the pump loads.
Go to S&P coils in Leicester UK and they have a free programme for calculating the RAC efficacy. Values of around 55% to 60% are theoretically possible but they will need more rows