I don't know about the "60% in 6 months" statistic, but it is pretty universally said not to store down products compressed and that doing so would negatively impact their effectiveness.

Here are some of the storage recommendations I found for sleeping bags:

MEC: Do not store a down sleeping bag or jacket in a stuff sack. The fibres or down plumules will compress.
REI: Do not store your bag compressed in its stuff sack as this will eventually damage the fill.
Seirra Designs: Never store your sleeping bag in a small stuff stack since, over time, this will reduce your sleeping bag's loft.

I store my bags in large cotton storage bags (something often supplied with down bags).

I don't know 100% about synthetic, but I've done the same with my nice synthetic over-bag too. Interestingly, it appears both the REI and Sierra Designs storage recommendations are intended to apply both to down & synthetic (at least they don't attempt to differentiate between the two).