I've been using the old Koflach Oxygen boots for the last few years, and I don't think these will be much different as far as flexibility. I put a pair of really stiff thermo liners in them, which helps a lot, and I'm planning on doing the same with the Nuptse's. I really like the setup I have right now, but the Oxygen's are hard to come by. Seems like the Nuptse is as close as I'm gonna get.
BCD (or anyone) You are welcome to my Koflach Superpipes, which are basically the same thing, except in a delightful purple coloration. They are size 11s.