I went from a 12 to a 18 for the same reason you are thinking of.
The ride got kind of bumpy when lightly loaded. On heavily loaded trips it was fine. Weh had a bitg debate about this here some years back, and the upshot of it was that a longer bar lengthens the leverage point of the bars, making them act stronger. I was going to see if I could adjust the bumpy ride out, but never got around to it (it really wasn't that bad), and got a new truck. When I set it up on the new truck, no bumpy ride. So I believe that it can be readjusted.