Lightweight you're looking at down. Just don't get it wet.
The wet down thing is a weird one.
No doubt that if you dipped a down and a synthetic bag in a stream, the synthetic would offer more warmth than the down.
If it's dripping wet though even synthetic bags won't offer anywhere near the insulation as when they're dry.
*edit just watched the vid you posted Andy, impressive but i wonder how much the waterproof fabric is responsible for keeping the warmth in (i.e, not allowing the insulation to wet out)
For hiking and the like (i.e. non water based activities) in my experience you'd struggle to get a down bag wet enough to lose a lot of it's insulation if you use some common sense.
Add the fact that many down bags these days (PHD included) come with a rainproof outer shell (not waterproof as the seams aren't sealed) and as i say you'd have to be going some to get the bag wet enough to lose it's insulation.
I wouldn't bivvy a down bag without a waterproof outer bivvy, but then i wouldn't with a synthetic bag either.
I've had my down bag outside the tent airing and lofting while i nipped for some water, only for it to bucket it down.
My bag was out in the type of rain i could only describe as biblical, for at least 10mins, shook it off, gave it a wipe down and i honestly can't say that it was any colder that night even though my bag is rated at 5c and it was about 5c during the night.
So although there is some truth to the wet down thing in my experience in real world type use it's not really a problem for those of us carrying out land based activities.
Should add that if i was canoeing or doing other water based activities in cold weather then i'd go for synthetic, but for land based activities if you use a bit of common sense down is fine.