Successful enough to win the majority of the over 60 amateur fights that I fought as an amateur & professional boxer. Plus another ten fights as an amateur & pro kickboxer.
I'm of the mind that the vast majority of the benefits that fighters say that they feel as a result of abstinence is psychological. And if that works for them, like many insist that it does, then by all means they should continue to practice what works for them. But a lot of fighters are so damned superstitious about it that it's bizarre.
I knew a fighter who was so mentally caught up in it that he actually lost a fight because of a silly wet dream that he had the night before a fight. He was convinced that because he orgasmed in his sleep that it weakened him. He pretty much had a panic attack about in the locker room & he kept insisting that his legs felt weak. And sure enough, he allowed his fears to become reality & he wound up getting beat up for a couple of rounds before quitting in his corner because he supposedly couldn't feel his legs.
Later on that night, I fought & I stopped my opponent in the first round. And I'd had a great night of sex the night before with my girlfriend.