In reply to badgerjockey:
Better to bivi at southern end of bealach a' garbh-choire, then head out and back for the first top without any kit, picking it up as you return. There's water just down the slope towards the lochain too.
BUT please return for all your bivi kit.
Personally I don't think there is any advantage in bivi-ing the night before, you'll sleep worse, walk in heavier and arguably not eat so well. Plus there's toileting issues.
Better to leave early, walk in slowly, aquire water en route, bag drop in a similar place for first summit, you can briefly bag drop for Dubh Mor and Alasdair too.
If you want an advantage, drop fluids and snacks at An Dorus the day before and recce just after there for an hour or so out and back over Mhadaidh.