In reply to stayfreejc:
I have an older model Alpkit Pipedream 400 probably lower specs and slightly heavier than similar Rab bags. It is in fact better in some ways than my old low temperature bag since the narrower bag with little space around the body, neck baffle and good hood closure seems to maximise efficiency (probably most reputable bags have these now).
Lowest temp use I am sure of -4C, old karrimat, silk liner, light clothing, tent....very warm.
Obviously anything which compresses the down either from above or within due to excess clothing will be counterproductive. Again obvious but a good compression bag is great for reducing the volume....if I ram extra items in with the sleeping bag I get more in the space (possibly self delusion!).