Using down trousers and jacket instead of a sleeping bag?

Not sure but suspect you might well actually have more material overall with the clothing, its certainly not an obvious saving. And you don't have the same sort of all body seal you've got with a bag. 

Much more viable for boosting sleeping bags (where there's room inside to let the clothing to still loft.).  

Include a little history in your walks. Pecsaetan - Ancient Derbyshire, Staffordshire and South Yorkshire - http://pecsaetan.weebly.com/

your metabolic rate falls while asleep so you get colder then. There is real logic in needing more insulation while asleep. It may not be as much as a thick down bag and down under you is compressed and of not much insulation value. Insulating your arms and legs separately is a waste of insulation with arms and legs together. A light top bag, insulation under and a down jacket might be the lightest system. It might be warmer to take your arms out of the jacket and pull the sleeves inside across your  chest to sleep too!

I use a platypus and a foam cooking cosy in a dry bag as a hot water bottle for no extra weight if our light sleep system appears inadequate on an unexpectedly cold night.

Think of gloves and mittens... mittens are lighter and warmer at the same basic spec.  Similarly, trousers will be heavier and cooler than a similar spec of bag.

Pete.

 It would depend on what time of year you are thinking about using this.

 If in winter you'd need such high rated clothing, a bag and lighter clothing whould weigh about the same, would also allow more options.

I have a pair of fleece trews and 100wt fleece pully for cold nights - combined with a lighter weight bag.

If it looks to be distinctly chilly, I have a pair of snugpak insulated trews and a belay jacket - but thats as much so, if I have to get up I am semi dressed

Daniel, that's what I have. I use down trousers and booties with keela belay jacket. I also use a silk mummy bag liner, I think that's important, stops air currents around you. No problem with this set up all last summer, total weight of night time gear is 532gr

for the winter I got a 100gr per sq mtr sleeping bag from poundstretchers, (£6.99)  cut it off at chest area, made 2 loops to go over the shoulders, ditched the silk liner and this set up did me for 2 night on Ben Nevis to -15 last January for a total weight of 948gr (in a Marmot alpinist bivi bag)

Have just finished my upgrade to 150gr per sq mtr Highlander soft tec mummy sleeping bag, used 2 weeks ago at -1 and no problem

Went for a mix of technology after reading Andy Kilpatrick on the same subject

Alpkit Pipedream 200 is the half bag you might be thinking of Robert, will be on sale this summer, it uses a gillet extension (unfilled material). This is important area when thinking about half bags, my own learning curve with loops (braces) under or over the jacket, or under the middle layers, a lot of heat dumped when going for a wee in the middle of the night. it's very important to have something to hold the bag up.

As always the ground insulation is ultra important,