Mark I have played with that permissions thing quite a bit since 7.X.X came out and it is pretty easy to use for mass permissions. It will do what I think you are looking for.... say 3 forums with this permissions set and 15 others with another type of permission set... it will do that... you just have to have one that is set up he way you want the permissions for the rest then use the Copy function to add it to all the others of the same permissions.
Dunny