Lots of European and Argentinian boxers entered the sport because they couldn't hack it in soccer. Quite a few of the West Africans as well.
Holyfield was considered a very good athlete. Hoever that doesnt mean he could ever draw a paycheck in a sport. Conversely, i have always heard that Tyson was slow of foot an clumsy when he tried basketball or football. Ali seemed really s l o w when he did the Superstars competition. I believe he got last in the 100 yard dash.
Yep. This whole transitive exercise in boxing to other sports and vice versa is tedious and fantastical. Outside of balance, reflexes and stamina, I can think of nothing absolutely essential in great amounts to make a championship boxer. So much of it is ingrained in the mental make-up of the practitioner. Actually, come to think of it, perhaps tennis is closest. And perhaps Floyd Mayweather would have been a great tennis player.