BF, you are throwing around the FP rating of the equipment and TBH, that has very little to do with how well they will perform in the cold.
The FP does indicate how much down it will take to fill a given space, but whether that space gets filled with 900+ down or 500 down has very little to do with the temp rating of the product. If you have a down bag with a 2" depth of down, the temp rating will be essentially the same for a properly filled bag, regardless of the FP rating of the down used. The difference comes in that it will take more of the lower FP to fill the bag properly and hence it will weigh a bit more and compact a bit less.
Better design/quality gear typically uses better down (Higher FP), but the performance difference is also largely attributable to better design and materials, as down is just a material used in conjunction with other materials in a design to help prevent heat loss .... so the point is, it is the thickness of the down layer that determines the temp performance ... the design of the gear has to allow the down to stay in that layer at the right thickness ... and finally, the amount of down to create that layer will vary with the fill power. better gear is trying to reduce weight and packed size, so better down makes sense, but without the other attributes it can be mostly wasted.
Brian