I actually do think Tyson Gay would be a medal contender at 400m if he focused on it.
44.89 in his base training phase in mid April is pretty fast.
44.5 pretty much makes you a medal contender these days and I have no reason to think he couldn't get there.
I generaly will give the nod to a 200 runner against an 800 runner over 400m if they are at the same level in their event.
Rudisha is a speed based 800 runner which makes it interesting. But he isn't as fast as Mark Everett was and Everett couldn't quite make it as an 800m runner.