I've noticed that a number of people mention hiking with lightweight wind shirts and baselayers for most conditions, but who occasionally get cold because of changing conditions, rests, etc. I'm normally a sweater or fleece guy, but i do see the advantages to the above system especially when wind is stronger and i plan on experimenting with this more lightweight system. Throwing over a lightweight insulation jacket (whether down, or synthetic) seems ok, but may increase warmth too much especially during exertion, plus you have to take your pack on and off to do it.
What about removable insulation panels, just for your front and core. This is what i was thinking: get some thinner Climashield Apex material, cut it to size your front core (maybe coming up to your neck some?), sew some DWR treated silk or other lightweight type material around it. Your back is already extra insulated from your pack (and most likely already sweating heavily most of the time), and so if you get cold, just whip this out from a side pocket and slip it under your wind shirt and over your baselayer. Maybe sew a couple of loops on the top of it and your winshirt so you can easily fasten it (and permanently tie some dyneema cord onto it). Such a small and thinner piece of insulation, covered by a very light fabric, won't had much weight at all, but might make a considerable difference comfort wise and more importantly save you time and effort as compared to putting on a jacket?
It also could do double duty as a pillow.

