With third Df he will match Roger/WB. I have no reason to believe other wise. He already has strongest paramecia and strongest Logia Df and if he gets strongest zoan then he might even surpass Roger/WB even if it's little.
I can see BB having all 3 haki types, with having even adv CoC.
At top awakened Gura+Awakened Paramecia makes him equal to the two letter.
With a third df(zoan) he will surpass them in my opinion.
Luffy vs BB will be about two guys surpassing Roger/Whitebeard.