I'll stick my neck out first and guess 280"...

As others have noted, a big bull in one area may not seem so big, in comparison, to a bull out of a hard to draw trophy area. When I got my first 6x6 many years ago with my rifle, I had it mounted because any 6x6 out of that OTC unit was a sight to behold and he only scores about 265". Then this year, as luck would have it I killed my best bull elk to date with my bow out of an OTC unit and he scores right at 330" and is fine bull for that area. But if I were to draw a tag in a premier unit in AZ., for instance, I may be looking for something in the 360" class because the odds are there that it can be done.
As also has been noted, 400" class elk are usually found on private property or in very hard to draw units and very few of them are killed, compared to the number of guys that would love to kill one. Heck, very few of us every even get a chance to lay eyes on such a beast, let alone have a chance to kill one.