I run 15x8's with my 35/12.50's. I wouldn't change a thing. I like being able to air down to about 10-15 psi and not worry about it. As for running a 15x10 offroad with 30 psi and airing down on a 15x8 to about 12 psi, it is not the same at all. With the tires aired down they will give you a much larger contact area, because they actually pancake a bit, resulting in an even larger contact area. Also, the wider your stance the happier you will be both onroad and off.