My guess is that if ABS if producing the increase in walks, they will come back down - at least part of the way, because pitchers with poor command exposed by ABS will either adjust or be replaced by guys that have better command but maybe give up a little more contact. Either way, whether via walks or contact, there are going to be more guys on base, which should lead to some increase in run scoring.
The other effect I would speculate about is the top of zone change. Now every player has a fixed top to his strike zone which is going to be the same no matter who the umpire. That gives the player the chance to learn his zone to greater accuracy than before ABS. That has to be a second aid to hitters on top of not having to cope as much with the outside breaking ball that fools both players and umps and so often gets called a strike.