I have to throw this out there for comparison, because I think it shuts the door on whether Lebron could "break 60" or not.
I was a 48 400m, 1:48 800m runner in college, haven't run, hell even exercised much in 5 years. Took a bet while drinking with buddies I couldn't lace 'em up and break 60. I'm now 40 lbs. heavier than my racing weight, mostly fat! I hit the 200m in 26 and cruised on in in 58.5, ok I was tightening up the last 20 meters or so.
I've heard Lebron called the fastest guy in the NBA down the floor by many people. Lots of guys quicker but his top end unmatched. Guy would run a 51 or better, without question. Runners, especially distance runners, don't be so self righteous. 400m is a sprint not a distance event, and we had guys come in straight off the football field and run 50 point high on our 4x4 relay. I've said this before, but go to Texas Relays and watch the HS division 4x400m relay and watch the dozens and dozens of pimply 16-17 year olds crank out 50~ quarters. It just takes a little natural speed and some guts, Lebron's got both of those.