"As the loft increases the amount they use decreases. "

Seems reasonable that if a jacket/bag/etc. were designed around the loft of down to be used, that would not be the case. Bigger baffles, loftier yet lighter down, more dead air, lighter yet warmer jacket. No?
I can certainly see that the SAME garment stuffed with 650 or 850 should insulate about the same. My TNF 700fp is 'overstuffed' in my opinion, which defeats the purpose of 700 fp. It's still warm as heck.