Jim is right. You need to have enough fleece to keep you warm while active in the worst conditions that you expect. The base insulation needs to be noncompressable.

800 down jackets are for wearing in camp.

650 down jackets are stadium coats. Ones that get beer spilled on them by the guy behind you at football games.

I want to try JAKs system of a big heavy wool sweater. I have checked the thrift stores a couple of times and no luck so far.

IMO down of any grade is not a good choice to wear while carrying a pack.

Synthetic insulation like the Patagucci MicroPuff is good camp wear, but it loses loft when compressed.

No practical experience, but I consider the ThermARest to be my emergency insulation. Wrap it around the body, put a layer on top, then allow to inflate a bit.