Re: .300 WM bullet choice
I think a minimum of 180 to 200 + grain bullets are great for the .300. They tend to have a high BC.
That said, the statement made may have a lot to do with magazine constraints on factory rifles. If you go too heavy, and have mag constraints, you will have to seat deep, which takes up powder space. As far as the short neck, that is not a hinderance IMHO.
If you don't have to seat deep, then you can go heavy.