He did plenty of training and dedicated a good portion of his everyday life to training. Like you said, he died so young and he left so many of his own questions unanswered that the world will never know how well he would do in competitive combat sports.
By that logic alone, it is impossible to say for certain that he would do well. While I find the view that "Bruce Lee is a myth... an actor only" a close-minded one, I find the view that "without a doubt, Bruce would own" just as ignorant. The big reason why these topics are never taken seriously is that the majority of the contributors are either on one extreme or other on the spectrum of things.
The truth of the matter is that this topic is all a big game of "what if" that can not be proven one way or the other. It's entertaining (in a slightly embarassing sense) that posters get so worked up over this topic.
While he was a phenomenal martial artist who was physically gifted, he was also an intellectual when it came to the workings of the human body. He was a training freak, much of because he was a live experiment of his own theories. While that is a given, there is no sound proof of his performance against a higher level of competition in a striking or grappling environment.
Heck, after working with Gene LeBell even he knew he had to incorporate a greater body of grappling techniques into the ever evolving cannon of "Jeet Kune Do." He knew it was something he had to work on. He knew it was something he was not as good at doing in comparison to throwing sidekicks or lead straights. Considering that, I doubt he would do well against a world-class grappler.
A little off topic: But by many of Gene LeBell's and Dan Inosanto's accounts, Lee loved to grapple. He just never did a lot of it on screen because it would have been too hard to portray the technique in a way that could be easily appreciated for the audience at that time. Not that it didn't stop him showing off a Crucifix Neck Crank on Samo in Enter the Dragon, a Guillotine Choke against Chuck Norris in Return of the Dragon, or a Bulldog Choke against Kareem Abdul-Jabar in Game of Death. His lack of grappling knowledge was something he admitted openly.
Does that in any way detract his accomplishments in my eyes? Nope, not at all. I definitely give him his due. I just cannot give him any more while maintaining my own intellectual integrity.