In reply to nickinscottishmountains:
I've had a look at a pair and they're not what I'd use for anything serious above 6000m. They don't have an inner like the Scarpa boots, being instead almost the same as the Arcteryx doubles ie a sort of insulated sock, which I tried and didn't find either particularly warm nor useful for more than a night or two. To my mind they're not really a true 'double', maybe a 1.5, and aimed more at over nights in the Alps than high alt stuff (despite the marketing).
Depends how you get along with this trend in skinny inner boots ala the G2SM's as well, something I also didn't get into. I like me a nice plush inner like Scarpa and TNF do, that shares a lot more of the boots insulation with the outer and lets me wear a thinner sock (that dries faster).
Positives; they're very nice quality and impressively light, the Boa's got it's advantages.
For what you're aiming for I'd personally use Scarpa or TNF's, depends on the climbing and how many nights above 5500m. I also don't think big doubles would hurt if it's north facing and later in Dec.