I love RAB sleeping bags, I have 3 of them, my husband has 2 of them, yet none of them where what we took on tour (well I started with a Rab neutrino 700? but it died 2 weeks into the tour - human error). They are an investment, but they are worth every penny when you can wake up in the morning with a layer of ice over the top of the sleeping bag and know you are toasty warm - that was -18C in the Lake District many many years ago. Looked after, used with a silk liner or washed professionally they last. I know, we have 2 of the old Rab Premeire 1100 bags. One was 2nd hand when we purchased it back in 1991, the other was purchased new (£395) back in 1995 I think it was. The younger of the two you can not tell it's age at all.
For what you are after, I would probably be looking at something similar to what we were using back in Turkey at the beginning of this year. I have a Marmot Pinnacle (comfort rated to -9C, extreme rating to -30C) used with a Exped downmat 7, silk liner and on the colder nights I simply put my thermals on, hats and gloves and slept in those - I prefer to have my head out of the sleeping bag unless it is seriously cold. We were living outdoors at the time and used to the cold, so that did us down to -10C without issues and I need to be warm at night to sleep (warm toes, cold nose rule). We never got the chance to test it to lower temperatures, but between the tent and the bag/mats + additional clothing we had available, we did not consider it much of an issue - sure I would have loved to have been carrying my big RAB sleeping bag but if I was, I would not have had room for anything else in my rear pannier at all.
PS - I have never had any great issues keeping a down bag dry - the only time I failed was in Greece when the bag was hanging over the bannister rails of the holiday home we had taken and a sudden rain storm came over blosing open the windows and raining directly onto the bag. Sure the bag gets damp but they are easy to dry out in the evening by simply putting over your legs whilst in the tent and they air naturally. We did this for 12 months, plus the odd hanging over a bush to air in the late afternoon and never needed to do anythign more serious and did not suffer any mold issues at all.