To my knowledge discipline/tactics/morale/combat ability are calculated for individual regiments...My MP group uses a mod that nerfs forts to 2/3/4/5 from 2/4/6/8. It helps the late game. We got the point where people pretty much refused to play 1700s+ since the fort system scales out of control late game.
I'm considering adding in-combat attrition to the mod now too, since every war devolves in to rotating doomstacks, with everyone stacking on the player that used offensive+quality to get a good general+discipline. Which gets a bit silly when you think about it, quantity players stacking their enormous troop count on to another player and effectively ignoring their low discipline+bad generals.
Well, that's what the devs say.