I think it's a given he could run in the 44's and most likely could run in the 43's and possibly even set a wr in the 400m. He's not too bulky to do that.
I doubt very seriously that he could ever break the 800m wr if he trained excusively for it because he is too bulky for that. May have a slim chance if was less bulky but probably doesn't have the physiology or the aptitude for it.
He isn't too bulky to break 1:50 for the 800m if he was training for it and could probably do it while just training for the 400m.
A 400m in 44.0 400m is worth about a 1:42:69 800m for an equivalent effort (http://myweb.lmu.edu/jmureika/track/Mercier/window7.html)
Since he has more of a 400m body than an 800m body it would be hard for him to do the 1:42:69 if not impossible even if he is training exclusively for the 800m (if body type remains the same). But, if he were training for the 800m, he could easily run a 1:50 800m based on the quality of his 400m. and would most likely be able to do it with just 400m training.
I'm sure he will never try to train exclusively for the 800m and I don't think he should try. I do hope he goes for the 400m record (long jump too) and maybe in his buildup for the 400m record attemp we could get to see him try an 800. I'm sure that would prove my argument.
By the way Paul McMullin ran a 3:54+ mile while being about 6'3" and weighing about 195lbs (http://www.iaaf.org/athletes/biographies/letter=m/country=usa/athcode=20030/index.html).
I'm done with this thread.