I've never climbed in super cold weather with leathers but from what I've heard and read, modern insulated leather boots and hybrids like the Scarpa Alpha are warm as hell. However, most of those dudes who have spent mad time in Alaska like Twight, House, et al. still swear by plastics and I would to. Its just a lot harder to fuck yourself with plastics and its nice to have a liner you can wear at the bivy and in your bag. The deal is to get some custom Raichle Thermoflex liners (not the ones for ski boots, but the ones specifically designed for plastic mountaineering boots) they run about $150 and are half the weight and about twice as warm as standard liners.