Have you more info on the G.50 test? afaik this fighter had only one type of propeller the Fiat Hamilton, italian give a speed of 470 km/h to FTH (engine at 2500 rpm and 790mm Hg)
Back to question all the fighter that can go to 300 mph or more are not the slowest. As already noted by Corsning the I-16 type 17 would be a ~265 mph at FTH, and it's probably the slowest, taking out the older type 4, that was much slower but at time of its introduction was also the fastest SS, SE, prop, MP with RLG
