I agree, think of ALL your clothes as a system. A lightweight insulating baselayer (wool is my preference, but to each his own) — top and bottom — would not only keep you warm on the trail if needed, but would add a lot of warmth around camp; those baselayers should be ~5-6oz each for top and bottom.
A full down jacket is likely to be overkill. If you want the insurance of a down layer, especially to keep you warm during stops or around camp, I'd go with the lightest Montbell vest you can get your hands on (mine is 5.0 oz, size M). People have them used here on the gear swap sometimes, or I got mine new on sale for less than $80. I'd also check geartrade.com.
Between a down vest and a windblock/rain layer, you should stay warm almost no matter what if you're moving at all. If you're not moving, you're probably sleeping for the night, and again, wearing all your clothes — including a baselayer and a down vest — should extend your comfortable sleeping range ~10 to 20 degrees.
For ~1 lb of total extra weight, the peace of mind for those three pieces of gear would be worth it for me, but I don't like to sleep cold — just makes me sore and grumpy the next day. I'll happily sleep on rocks and roots, but just don't make me sleep cold. People all have their different comforts on the trail. I'll happily leave a stove at home and bring a colder-rated sleeping bag if I need to.
With the down/baselayers, you could make up a lot of the extra weight with a lighter sleeping bag. And of course, once it turns summer, just ship that pound home and you're golden.