If I accept your premise, then you can walk over with 1-barter using the peripheral moves, and auto-pass any manipulation roll made against that mob with a 10+. If the rule trumps real, then it doesn't matter that those oddments barter split between them is worth less then shit. In my opinion, your example and this one are made on the same merits.
If your guy goes aggro on the gang, and shit does break out, sure. He might be able to kill some of them, but he's also going to SUFFER HARM that might not go away very easily, he's probably outnumbered, if any of them got guns, you're looking at 3-4 harm fast. Not to mention damage to his bar, his reputation, and other causalities from people in the bar that weren't involved but got caught up in the middle. Being allowed to attack a group, and doing so without consequence are completely different. It isn't free, that group target move. Even if they don't force your hand with that go aggro, they're going to remember those threats. That's a good deal of animosity to deal with (and a large number of recipients to deal with it from) for a SUCCESSFUL move.
What I'm getting at is, make it seem real, is essential. You can both make it awesome and real at the same time. Manipulation doesn't carry the same... consequences as does those group HARD rolls. At BEST, they'd being doing their manipulation under fire of multiple opinions. Lastly, I agree that manipulation revolves around the concept of leverage, specifically the non-violent kind (not words backed up by guns, but words backed up by words), and what might work for one person might need to be different to work with another. If it needs to be different, then it cannot work for both. Missing that is likely to resort to violence, sure, but hitting it, even on a partial, won't. It's almost a freebie in this way, if you give it that kind of power. Although I am humored at the thought of needing to make a different promise to every person in the gang/mob present (which highlights the difficulty I'm suggesting gets whitewashed otherwise).
It would simply be better all the way around to force the person to actually read the situation correctly. Then zooming into the mob as more then one roll, but more-so as one entire scene, as they say how they get to that person, what they say to them, have an exchange and attempt to convince them to back down. It becomes personal, the mob becomes more then just a large clump of things, they become a dangerous landscape you're attempting to undermine. This is a great opportunity for suspense.
Also, the Hocus might not be about converts in his use of frenzy, no part of the move forces that to be true. That's just one possibility.