I've found that I'm most comfortable with my highbacks at 0 degrees, in line with the bindings. I had my forward lean set to 3 degrees on my old 390's and I currently have my new Boss's set to 4, since there is an extra degree of adjustment on the Boss's.
it seems strange to me that you would want the lesser canted footbed to the rear for pow. The more canting the further back you can shift your weight before your foot gets past flat....I am sure the engineer's at rome have thought about this and I am an idiot.... it just seems counter intuitive.
on the other hand maybe a greater angle in the back vs the front shifts you to a natural forward lean (from tail towards tip) which causes powder riding to become harder without shifting backwards.
If you cant your front foot more relative to your back foot, in theory, it would cause your hips to naturally settle rearward. IMO, in practice, it's not enough to matter.
I ride my bindings 15/-15 with the highbacks rotated about 2/3 of the max rotation.