I don't understand the first part of your comment: surely tractive effort is a theoretical figure, based on cylinder dimensions and boiler pressure, and inversely to driving wheel diameter, so the exhaust has nothing to do with that. What a correctly designed exhaust should do is enhance the free flow of steam from the cylinders, and allow the fire to "draw" properly, thus making for a freer-running locomotive. Sadly, even into the later stages of steam loco design, it could be got wrong.