Thanks for the feedback everyone. I'm going to try stuffing my existing down bag into my pack without a stuff sack but I still want to figure out how much smaller volume I can get with a new ultralite bag in a compression sack. Does anyone have a photo and/or dimensions of their WM Ultralite or FF Swallow or Hummingbird in a tight compression sack?
The Western Mountaineering website FAQs recommends "a compression sack that has a compressed volume of half the included stuff sack" and also states "though there are no disadvantages to temporarily compressing your bag".