I was just wondering what class gets the most out of the gnome trait expansive mind, that increasing their primary resource
Well, nobody gets very much out of it.
Famously, Arcane Mage - but not either of the other Mage specs - is the spec that has to manage its resource most carefully. But 5%? That might get you an extra Arcane Blast every minute or two. The other mage specs barely remember they need Mana at all.
Rogue and Hunter are often scraping for a little extra Energy or Focus, so that might be a small plus for them.
Any healer spec will happily slurp up every drop of Mana they can get, so 5% extra is 5% extra if you are going to heal.
I don’t think there is any other spec that will really notice.