byeplane, the BAe literature says the high wing gives 4% more lift for the available wing area, the flaps fit snugly against the fuselage which makes them more efficient (not quantified), the tail plane is out of the wash which makes it more effective, it is also further aft being on top of the tail and acts as an end plate for the fin which means the fin is more effective and can be smaller hence less drag. All of these are small advantages that would add up to better performance in general.