Of course A-level athlete is a thing, and if Lebron had devoted his time towards combat sports instead of basketball, his athleticism (explosiveness, quickness, strength, agility, finesse etc. etc.) would of course have given him ELITE skills. Undoubtedly. But how would he handle getting punched? How is his pain-threshold? Does he have a chin? Those are important aspects of MMA we can't know.
Tyron Woodley, Henry Cejudo, Cody Garbrandt and Demetrious Johnson are guys I personally would categorise as A-level athletes. Guys who could probably fare well in a lot of sports, had they been devoted to that specific sport from an early age. Athletic, agile, great ability to grasp and master techniques, "sees" the fight (vision/IQ). However, the thing about MMA is that you don't really have to be an A-level athlete to be elite. You hit REALLY hard, but not otherwise you're not really quick? Fine, you have a solid base to build around. You have really fast hand, but again, not that quick otherwise? Fine, solid base. Really good timing and precision, but not powerful, explosive or fast? Fine, solid base. In MMA, if you just know your shit, know your techniques and have that toughness and determination, you can get really far being an F-level athlete.
I really believe though (given they could manage taking damage and had the "fight" switched to "on" in their fight-or -flight-response), that guys like Lebron James, Michael Jordan, Cristiano Ronaldo and Jim Brown would/should have made fantastic fighters, had they started in some form of combat sports from an early age. That kind of athleticism, determination and work ethic is really the optimal base for a champion (again, they still need toughness in regards to taking punches). And the IQ they've shown in their specific sports, I really believe it will translate to MMA/fighting, as I believe it would have translated to any other sport, if they had developed it in that area from an early age.