A sleeping bag does take time to warm up, however, after warmed you have less surface area to loose heat. It may take 2 hours to warm a bag, but then you have 6 hours to loose heat. A sleeping bag has better warmth for weight - less nylon for the given amount of down. To get the maximum advantage out of a bag, you have to be willing to cinch it up with just hour nose hanging out. A lot of people cannot sleep this way. My husband uses a half bag and down jacket because he is claustrophobic. I do envy him when he can get "out of bed" in the morning and keep on the warm jacket!

Another thing to consider is that any sleep system should allow you to "survive" in 99.9% of conditions you may face. "Survive" does not equal "comfort". I think if you buy into the light weight concept you realize that occasionally you will be uncomfortable because of your gear, however I do not compromise "survival". And the line not to cross is really hard to establish without experience. The sleep in the backyard idea is a good one to test your own personal reaction to your proposed sleep system. I have spent a few nights halfway up a mountain with only my day gear and teeth chattering. Nothing is as welcome as seeing the sun peek over the ridge!