How effective are 400 gr. bullets in the .458 Win Mag?

As I understand it there is some controversy about the 458 because of it's lack of case capacity, so that makes me wonder: why not use 400 gr. bullets?

Is sectional density so important that a low number negates the effectiveness of a 400 grain bullet? After all, 400 grains is still 400 grains...

What's the scoop?