Who said he'll actually use the ability against the person? He got Shanks without it, imagine what will happen if he used the ability around the surrounding plus his own combat ability. That can seriously own. Take into account he didn't really master the ability too.
And yeah, I know I'm late on the reply. Couldn't help but answering.
Anyways, cut the Shichibukai level crap. THEY DON'T HAVE ONE. It's a force that if gathered, equals the Yonkou, or gathered+Admirals, or anything like that that's maintaining the balance. You can see Mihawk for perfect example. He might beat Shanks' in a fair fight, but hey, he's a shitty Shichibukai.
Also, we don't know anything about WB or Doflamingo, so how can you say anything about a fight? Hell, we can't even point Ace's strength other then the 2nd division captain thing, and that's not enough.