And if you look at it, BB's df is a great match-up to Luffy's! The gura gura nomi won't affect Luffy like normal people would as he is made of rubber. Luffy will be able to take the damage better because of that. Luffy is physically a beast too so he would be able to match BB zoan body/strength/speed just like he did with Lucci. BB will have to use his yami yami nomi to neutralize Luffy's df to inflict real damage which would open up his defense and Luffy would sneak in an attack too just like Ace did. It will go down to whose gonna be able to take a hit better.
If I could make up a match-up like that, tells us that BB having 3 df ain't overkill. Sure he will improve by then and may have come up with combo attacks but so does Luffy.