Just think about a 12.5" vs 10.5" wide tire. There is more rubber. This means more spinning mass. At a given speed, which tire do you think it would take more energy to bring to a stop. The one with more mass of course. This implies that you would have to brake harder to stop in the same distance as with the smaller tires. Additionally, you'll spend more $$ on gas since there is going to be greater rolling resistance.
One piece of advice. I am running trXus MTs on TJ Canyon 15x8 rims. I have blown the bead 3 times on 2 different tires in a total of three runs. This is while running at 16 PSI. Granted, the conditions were not optimal, but none the less, I've had problems. If you are going to continue running the 15x8 rims, then I would go with a wider tire, i.e. the 12.5 inchers.