I have the spidertrax rear disc set up on my sammy running 33's and it brakes soooooo much better than my stock rear drums did.  i would NOT run without a proportioning valve though.  You still need 70% of your braking up front and a prop vavle can adjust it so its right and safe.  And yes discs work better crossing water,  no fade at all. ÂÂ
Well, I think the majority of my braking comes from the front anyway. Reason being, my LSPV is in the stock location. I have yet to touch it. I have around 2" of lift, so, Compensating for the lighter rear end (Or so that is how the LSPV sees it), there is less pressure going to the rear brakes.
I'll try it without a PV first, if I don't like it, Summit Racing is about a 20 min drive from my house.