As far as the world power discussion is going, in the translation I read, it didn't even state the Admirals exclusively as a world power, it stated that the Yonkou, the Shichibukai, and the Marine HQ formed a balance, meaning that even 3 full-fledged admirals aren't a world power on their own, but with their fleets they are, I guess.
I always considered it 4 unified Yonkou= 7 unified Shichibukai= Marine HQ, but we also have to consider that neither the Yonkou or the Shichibukai have any intention of uniting, so that's always probably throwing the balance off. It may not be correct, but I've always assumed that the WG has realized that not every Shichibukai will act in their favor all the time because I don't think they're so naive.
I've also been wondering that if they do decide to send an admiral to aid the Tenryuubito, will it further disrupt the balance and give WB an edge? If nothing else I'm sure it will give him a better shot at rescuing Ace, since 2 is better than 3.
In any case, I could always be wrong. Between Oda's unpredictability and the validity of the translation I was reading, I reckon it's all up in the air at this point.