WRT the F27 it was originally designed to replace the DC 3, the use of the high wing allowed a lower fuselage "lip height allowing rapid on/offload w/o extra ground equipment. if you look at the HS748 you can see that it would have been possible to go low wing, with the same engines. however forklifts are necessary to load/unload. one of the reasons for the ATR's higher speeds are a higher wing loading, and a smaller fuselage, and more streamlined front end. The F27 vs the HS748 F27 is 10-15 kts faster, but the 748 will go into a 3000' strip w/ a full load. having flown both of them I prefer the 748 for it's human factors/systems. though I appreciated the "drag brakes" on the Fairchild version. Too bad they hadn't collaborated as a true high wing Hawker would still be in production with the Pratts and the ATR wouldn't have seen the light of day. Also the Darts if I remember correctly had a 10-1 reduction ratio and would have a prop speed of 1400-1500 rpm