Crosby won two TTs on one , the 81 TTF1 race and the 81 Senior Classic. He should have won the 1980 F1 but for Grants Honda 'fuel tank controversy'. He was also the first rider to complete a six lap TT in under two hours after being made to start at the back of the grid and overtaking nearly the entire field on the road. He also won the TTF1 World Championship on one in 80 and 81 and the Suzuka 8hr on an endurance version, (Yes I know thats not in the IOM either!)
McElnea won TTs on one in 83 and 84 in the Men Only colour scheme despite also doing 500cc Grand Prix.
I think you could call it a sucessful bike.
Incidently it was never 750cc, there was no need as F1 was 1000cc until 1985.