If anything, this chapter does not downgrade BB crew, rather, it puts Ace on equal terms with the most powerfull guys in the OP world. Surely, right now he is a rookie in this realm, but just the fact that WB confides in him for a mission such as this, Shanks seeing Ace as a promising guy (although trying to protect him from what he knows to be a bigger shark) and BB telling his crew to back off shows that Ace is, indeed, in the bottom or middle of the top league.
That, howerver, does not change the fact that guys like Garp, Aokiji, Sengoku, Mihawk, Kizaru, WB, Shanks, Dragon and a few others yet to be revealed are the parameters nowadays.
And although I agree Ruffy´s crew is somewhere nearer BB´s after the Enies Lobby incident, it´s exageration to say they could tie with them. I guess they would be easily owned at this point, actually. There is a progression to be followed in the Manga and even Usopp, who is the fartest from his direct antagonist in BB crew in terms of power today, will probably follow and respect this rule as the story goes on.
Anyway, about the chapter iself, I must say it was really nice to see how well ODA developed BB ambitions and reasons. And BB is not the typical villain in a manga, neither in design nor in personality. Him taking Ace´s atack on purpose was quite the sight. I barely can wait to see next chapter ownage.