350/3500 series pickups - any improvement to the brakes?

I'm pretty sure most of the reason the heaviest-duty SRWs are going to 18s isn't for brake clearance (the 17s are big enough for that). It's because 18" tires tend to have 4-500 lbs of additional capacity per tire. While you can find 17s that are rated for nearly 4k pounds, almost all of the 18s seem to be rated for 36xx at least, which is more than the 3195 that most factory 17s are rated for.

For duallies this doesn't matter as you hit the capacity of the rear axle before you will overload the four tires.