To be quite frank though, if you are getting border aggression, your game has gone on for too long and you've managed to piss the AI off too much. It's a sort of indication that you might've screwed up somewhere. I've never seen border aggression happen ever, except when border aggression was first being implemented and it was a bit overzealous 
In my book, play on difficulty 7 and play subpar so that the AI gets strong enough to be able to actually be a fairly big threat like it would on higher difficulties. Play chicken with it!
