A couple of friends are running 40' s on standard suspension with no issues. One of them is running Mickey Thompson tires and it rubs a little inside the wheel well, his truck was not armored (don't remember his model) the other trucks are heavy armored and i believe that makes a difference. They claim to gain 5-7 MPH.
I am thinking about going 40 as well and keeping my 37ts for my 1151A1, only reason is my 1123 is lifted 2" due to the turbo and i like the closer to tires look than the lifted look, but the tires are pricey for a test, all i find are MT's and i am afraid they will be too noisy. I am running Toyo's open country AT and they have been super in every aspect.