keep upping the adaptability and try a +5c rated synthetic with +5c rather down bag - covers all your bases and warmer in winter than a single either down or synth (deals with moisture better, traps more dead air).
a few nights a go i used just this to about -10c and spilled about 500ml of water onto it - would have been nasty if just a down, but the synth handled it nicely, and much warmer than just a synth alone.
In reply to Capt. Spacey: Mountain Equipment Starlight IVs are pretty good, rated down to -14C (comfort). Pack size 40 x 28cm. Had an ME synthetic for years & it stood up to heavy use brilliantly (once worked out that I'd spent a year and a half of my life physically inside it!)