In reply to stratandrew:
> Maybe. Perhaps this is true most of the time but a few manufacturers have bucked this. Sacs by Crux AK47X 47+5 (1240gram)...
But 1.2 kg is hardly 'light'... The Virga 2 (between 50 to 58l, depending on the 3 back sizes it cames in) is around 0.5 to 0.6 kg. And good upto ~12kg or so (at least that's what I carried comfortably).
Arc'Teryx Alpha FL 45 (which is might be a tad too small for the OP, if he also doesn't upgrade the 'big things' first), weighs in at 0.6kg.
And a more suited Alpha AR 55 is in the same ball park as the Crux (~1.2 kg or so). But does have more bells and features.
That being said, the I do know that people have been rather happy with their Crux sacks, but from their range, I'd prolly suggest the RK50 (1.1 kg). The main difference to the more lighter weight options is the fabric on the Crux-sacks, so they'll be less prone to wear.