I keep thinking about this as well. Unusually I got a train from Bolton to Piccadilly in the rush hour the other Thursday morning. It was 2*156 from Buckshaw Parkway. So 92m in length. This is a prime candidate to change to a 319 when they become available.
So the train will be shorter, admittedly with 2 less cabs, a 3+2 layout and wider doors, but I am not certain that it will increase capacity significantly. Needless to say it left Bolton full to bursting.
Going forward I know there could be more trains (but as you tend to queue into Salford Crescent I am not sure how many), so the trains need maximising in length, with Salford Crescent (and I imagine those more knowledgeable could highlight other points on the route) limited to six carriage use, the only sensible increases world be to 2 * 323.