Hi, just found this, having posted to ask if anyone has any idea if the Hard race ones are any good should have gone for the search first!
I, like you, was wondering what the difference it being hollow would make to the stiffness.
This companylists them as being 62% stiffer than stock, which would make sense as your Whiteline comparison chart states that 19-22mm change in solid bar would be 80% stiffer.
From a physics point of view: if it is relatively thick walled tube, that would make sense, as the central core isn't really doing as much as the sides when looking at torsional strength.