Hi there,
I'm no AW guru, but here's my 2-barter on the matter. Perhaps somebody more experienced can correct me on any (or all) of what I have to say.
Players frequently Opening Their Brains is fantastic! Every time somebody Opens Their Brain, they're handing you an opportunity on a golden platter to barf forth apocalyptica. Do so! If they're Opening every scene, then do so every scene! They'll advance quicker than they would normally, but that's fine. Uneven advancement isn't nearly as much of an issue in AW as it is in d20. Unlike d20, the key thing in AW isn't the advancement process itself, it's the fiction required to get from A to B. The endpoint (advancement) is just incidental. If that makes sense.
If you're worried about super-fast advancement, though, feel free to highlight their weak stats. Sure, the Gunlugger can keep rolling their highlighted Hot (or Weird) at every opportunity, but probabilistically speaking, that's going to trigger a lot of Hard Moves. And you can have a lot of fun with Hard Moves...
If their blatant XP-grabbing gets truly egregious, then do the obvious: chat with the players about how you feel they give more of a shit about the XP gain than the move itself.
Basically: no, there's no limit except that of basic common sense.
Ah, so it's coming to light now. I guessed it was my Pathfinder GM senses that was causing an issue and not the AP System. My group is loving the AP World system and we've put the Pathfinder game on hold to play out the AP Game for a bit.
I saw in the Roll20 Presents AP World that each PC saw the Malestorm in a different way, is that something normally seen in games?