I don't get anything, err, much of anything from board presses since my sticking is off of the chest. I don't get much from a parallel box, but I do get good carryover from low box squats to both my squat and deadlift. However, they take a big toll on my hips, so I have to be careful (the same reason I don't do sumo too much anymore).
Well I know cambered carries over for much like deficite pulls do. Im doing 2 board presses now so Ill see if I get a carry over since my next lift is bench in my rotation and Ill get to truely see whats helping and whats not. I get about a 50lb carryover from a just below parallel box squat to my free squat. And Im starting to really enjoy sumo lately.