I slept out on the summit of Mount Kenya in an old army sleeping bag rated nowhere near -20C and survived, that was 35 years ago mind.
I've a Rab Ascent 500 and only use it in winter as I find it too hot the rest of the year. It weighs just over a kilo, the 900 is 1530g the Andes 1000 is 1625g so a saving of 95g at a price difference of £370.
The recommended lowest temperatures are only a very rough guide I wouldn't worry too much about a 2 degree difference, if your cold at night put more cloths on.
I've not tried the Andes range bags but I have tried some of the other Rab high end, very light bags and find they save weight by cutting them very tight. The Ascent range are cut with quite a bit more room so there's room for extra cloths and still won't feel like you've been tied up.