I'm sure that if there was MMA like there is today back in Bruce's day, and Bruce had a few years to train, he'd be at the top of sport in his weight class. There isn't much ground fighting in Bruce's JKD, only basic judo and wrestling. His JKD wasn't complete, and Bruce knew it wasn't finished.
I think the only things holding him back given proper training would be a lack of motivation to compete and maybe his chin (I've heard that some of his training partners comment on how Bruce thought his chin was small or something). He didn't enter any kickboxing/boxing competitions because he wanted to develop a style, not necessarily prove a point by sport. He coached many champs though (EDIT: though for most he wasn't their primary/first trainer).
I think the only things holding him back given proper training would be a lack of motivation to compete and maybe his chin (I've heard that some of his training partners comment on how Bruce thought his chin was small or something). He didn't enter any kickboxing/boxing competitions because he wanted to develop a style, not necessarily prove a point by sport. He coached many champs though (EDIT: though for most he wasn't their primary/first trainer).