wow, finally got to the end of the thread!
not to beat a dead horse here, but i personally would get massive thigh burn and my sessions were really short because i was exhausted, but there was still plenty of wind and powder, i was pissed. bindings were wide with duck stance.
i remember reading a thread somewhere, (not here) to remove the high backs, so this winter i tried it, and i am able to ride more than 10x as long, with little to no soreness at days end. edging and going upwind were never a problem, and the boot stiffness more than sufficed for support.
reading through all the posts at once, it seems some people really don't like it without the high backs; and i read some good arguments about how snowkiting and snowboarding are not alike, just like kitesurfing and wakeboarding are not alike; neither are snowkiting flat and snowkiting on hills.
think about why you need high back support on a mountain. well, the surface is not flat, it slopes down at an angle, and you have to ride down this slope holding an edge a lot of the time. i guess without the bindings and high backs, your foot would have the tendency to flatten against the ground, and catch a toeside edge, yikes! your body has to fight to keep balanced (board level) against the slope. on a landboard, kiteboard, or snowboard on a flat lake, you only have to lift up a little bit, and leaning back against the kite facilitates this a bit too.
for snowkiters who kite hilly terrain (western North america, the alps, etc.), maybe it's better to leave the high backs in, because otherwise, maybe your legs feel fatigue from trying to edge too hard?
i imagine a landboarder, who normally has no bindings, but normally rides flat areas, might be uncomfortable become fatigued trying to keep the board level on hilly terrain. maybe a landboarder who rides hilly areas can shed light on this.
one thing i have to say is it works wonders taking the high back off where i ride, which is flat frozen lakes, maybe when i go snowkite out west this season, and i'm in hilly areas, the high backs may bother me less, but quite frankly, i think the forward lean and stiffness of my snowboard boot alone are enough that i wouldn't need a high back even then.
basically, if you haven't tried it, don't just assume that it's not the right thing to do. try it and see if it's better for you. you got people like motz, who was all against it, who now prefers no high backs.