So I have 5 full days on them now. First day with the medium bushings, next three with the soft and the highbacks on and then today went with the soft and no highback. The conditions were probably the worst you could want for that set up, really icy and hardly any features open. I have to say it was still super fun. The binding responds just as well without the high back and I was riding them in a two year old pair of Northwave Freedoms that are a very soft boot.
Thanks for your input. I asked questions earlier in the thread regarding going highbackless and it seems no one rode it that way. With the king pin design it seems theoretically you don't really need highbacks.