I too would never leave down compressed. The feathers, while bendable, would eventually break down overtime under heavy load. The loft itself, which is measured for effectiveness, gets damaged through compression. But don't take my word for it, here is the International Down and Feather Laboratory and Institute, the global leader in filled textile testing. It states (404 Not Found
"Why do subsequent fill power measurements drop?
Compression, shipment, storage, and finished product assembly cause fill power to drop. Comforters, pillows, and even bulk down may drop 5-20%. Jackets and sleeping bags may drop 10-40%."
There are tests to confirm that leaving something compressed will reduce the fill power of the garment/sleeping bag.
"Why do subsequent fill power measurements drop?
Compression, shipment, storage, and finished product assembly cause fill power to drop. Comforters, pillows, and even bulk down may drop 5-20%. Jackets and sleeping bags may drop 10-40%."
There are tests to confirm that leaving something compressed will reduce the fill power of the garment/sleeping bag.