In reply to Burly1973:
My PhD bag is rated -10 degc, i find that its a bit variable comfort wise, sometimes i'm warm -5 to -10 degc (beyond -7 often gets a bit chilly), other times not. Nothing getting up (if I force myself) for a pee and eating a chocolate bar can't solve. Obviously warmth is not just a function of the bag but also metabolism, humidity, & fatigue ect.
Having said that my older Rab is rated -20degc and I get chilly beyond -15degc in it.
You obviously have a higher metabolic rate than me, I wouldn't take my partners standard minimus to -5 degc, while her made to measure diamir is very good its too small for me (mores the pity).
At -10dec and below, shoulder baffles, contoured hoods, zips (for venting), zip baffles, shaped feet and a wider cut all start to make a marked difference to comfort between a hard core lightweight bag, and a long trip livability morale booster.
Post edited at 13:39