We tend to limit gaining XP from a highlighted stat to once per scene, though the goal is to keep scenes pretty short such that this isn't too much of an issue. But we found that some characters (especially the Brainer, who uses Weird for basically everything) were advancing really really quickly if their Weird was highlighted.
As for using stat-substitution moves, I would be inclined to only allow XP if the highlighted stat is rolled. So if I'm playing a Brainer with Unnatural Lust Transfixion and my Hot is highlighted, what that is telling me is that if I want to turn this person into a meat-puppet through seduction, sure, I can use my Weird and I will likely be successful. But if I want to learn something (i.e. gain experience), maybe I should try it the old fashioned way and actually connect with the person on a more human level. Just because you have a stat-substitution move doesn't mean you have to use it.
And remember, to do it, do it. You can't just say, "Oh, yeah, I totally Go Aggro on this guy to get him out of my way." You have to describe your actions, you have to add to the fiction. And if you do have a stat substitution move, I want you to actually describe how you are doing what it is that you are doing that uses the substituted stat. Playing a Battlebabe with Ice Cold? Tell me what it is that you're doing that's so Cool when you Go Aggro on someone. If the answer is, "Uh, I stick my gun in his face and tell him to do what I want," then I'm likely to argue that you're doing it using Hard, not Cool. But if the answer is, "Without a word I draw my katana using flawless iai, cut the belt of his holster straight through the buckle, then perform a perfect flourish of chiburi and noto, returning my sword to its scabbard before his gunbelt hits the floor. With my hand still on the hilt I say, 'We will be entering this establishment without further interference'," then hells yeah you can use your Cool! And if it is your Cool that's highlighted, then mark experience because you actually did something Cool.
What stat highlighting means to me is not that someone wants to see you do a certain thing, but rather that they want to see you be a certain way. They want to see that aspect of your character. By highlighting your Cool, they want to see how you rise above stress, how you are graceful under pressure, or how you impress people. By highlighting your Hard, they want to see you be aggressive or strong-willed. Hitting your Hot means they want to see you connect with people, to see you as a social animal. Highlighting your Sharp means they want to see your introspective side, or see you think before you act. And by pegging your Weird they want to see your spiritual side, want you to reveal just a little bit of your soul.