These trucks are tuned for 91 octane, technically. I would definitely try 89 and see if there's much of a difference. Because of the tune for 91, the engine probably only advances timing far enough to utilize it. 93, being harder to ignite, is going to be less efficient because the engine can't properly compensate.
I, personally, have found the opposite to be true. I seem to have more power all around with higher octane (we can't get 91 here, only 93 which I consider overkil/is higher than recommendedl, so I run 89). 87 causes some funny idle issues and makes the truck feel a bit wimpier. My gas mileage seems to be abysmal no matter what I do...