OK, so let's back up a bit to what's already been implemented, is already available and proven to work.
You create a profile. This is a set of permissions that applies in a board. You just set which boards you want to use this. Job done. This is even the case in SimpleDesk though it's an order of magnitude bigger.
No need for making a whole new level of permissions, complexity and confusion for the sake of saving time in the minority of cases. Not everyone is building huge forums that have tens of categories and dozens of boards.
Performance wise, this does seem more efficiant
How can it possibly be more efficient if it has to look in 3 places (outside boards, inside cats, inside boards) for permissions instead of 2 (outside boards, inside boards)?
As for not being an attack on intelligence, maybe not, but I'd think that maybe I'd have learnt something in 5 years as a forum admin, that I used when building my own systems, that I'd be using here. I'd also assume that I wouldn't retry things I've already tried hoping for a different result... you've made the suggestion, which is awesome, except that experience trumps ideas in this case.