This sounds like a good backyard project for next fall. Keep sleeping in the backyard as temperatures cool toward winter, and you'll soon know how cold you can go with the jacket and pants alone. (You might want to add down booties and mittens; cold hands and feet will keep you awake all night. Cursing.)

I'm not sure I'd trust jacket and pants alone, myself - but that's just me. I do know that I use my jacket and pants as part of a sleeping system, and carry a lighter sleeping bag than I used to as a result. I used to carry a 15 or 20 degree bag; now I carry a 30 degree bag, and use the down garments inside the bag to extend its range for the stray 20-degree night I might encounter. (As I'm getting older, I'm starting to avoid sub-freezing temperatures for backpacking, though I'll still go out for a night of car, or near-car, camping.)

I don't save as much weight as omitting the bag entirely, but using the clothes has saved me some weight - probably a quarter to a half pound, with the 850-fill bags I like; lower fill ratings might produce greater savings.

Interesting idea, but play with it at home first.