The answer depends on the quality of the down and the type of synthetic insulation you have in mind. Your LaFuma 800 uses 650 fill power down. With higher fill power down your bag could weigh less and still have the same rating (I notice your bag is rated at 30 degrees) and the converse for lower fill power down. Temperature ratings are unreliable indicators of comfort but they are at least somewhat consistent between bags. So you should compare your bag to a 30 degree synthetic bag. The LaFuma Extreme 1000 is a synthetic 30 degree bag. It weighs 36 ounces. So that should give you some idea. Of cours the comparison is not really fair because the two bags aren't exactly the same length.