The conventional wisdom among backpackers is that storing stuffed leads to a loss of loft (insulation thickness) and therefore warmth, over time. the best practice is considered to be storing it flat, under a bed or something. That goes for self-inflating pads, as well. Manufacturers usually advise against storing stuffed.

Backpacking stores, such as REI, sell very roomy and breathable cotton or nylon storage bags big enough to hold a bag without compression. That's what I use for my "good" bags, with the appropriate stuff sack inside. A cheapo bag stays stuffed and in the car over winter.

My team keeps our gov't issue bags (warm and intermediate) and bivi sacks stuffed and stacked in shipping boxes, ready for deployment. I've never used them in cold, but they seem all right. If your bugout will be by car, maybe the storage bags are good enough to grab'n'go until you have a chance to stuff 'em?