I well understand that feeling.
I have been looking around at stuff for the past couple days. (There isn't a lot to do around here on cold winter days...) and it seems that the various testing is likely about the best summary you will find.
800+FP down is lighter and warmer if it is fairly dry, but degrades guicker if damp or wet than 700fp down. I will not dispute these findings, indeed, they are far more rigorous that I have ever done. I suspect that loss of loft vs fill weight does cause some loss of insulating value, but this is not linear as you would expect from other poasts on overall performance of insulation. Compression of any air pockets inside the down would lead to increased efficiency of the insulating value (by creating smaller air pockekets, perhaps more of them due to the fibers rolling) without greatly increasing the conductance of heat by the denser materials making up the fill. So, some compression would likely be OK, with only a slight loss of thermal insulation. Anyway, just a guess...
Soo, that leaves the conditions of "where" you do most of your hiking. If in the northern parts of the US, a mix of rain and dry weather, perhaps a 700FP bag might be a better choice, based on weight (good weight), excelent insulation, good dampness resistance, and fair cost.
An 800+fp down under the same conditions will have excelent weight, excelent insulation, poor dampness resistance, poor cost. But, 800+fp would be a better choice for dryer conditions, generally.