There IS no science on down fill power, just regulatory standards which were relaxed under the Bush administration. Therefore, 900 cubic inch down is 2005's 800 cubin inch down.
Fill power is measured by how much space a precisely weighed sample of down will take up when it has a certain weight on top. Under the old standard, the down was compressed under the weight (a piston) and released. Its fill power was whatever volume the down returned to. This gives a realistic picture of how a down garment will loft when removed from a stuff sack. Under the Bush standards, the weight is placed on the down and allowed to settle. There is no compression step. The level to which the weight settles is taken as the fill power. Obviously, this gives a higher value - which is just what some big manufacturers wanted.
Down testing is supposed to be done by independent labs. It is supposed to be done scientifically. The tests are supposed to be consistent. If all criteria are met, you can compare different down regardless of what test is used. However, if you design a garment to hold X oz. of 900 cubic inch down, it will be underfilled. If you calculate your volume based to 800 cubic inches per ounce, you ought to have the right fill.
About personal weight. It is still better to go as light as possible because you will travel faster and farther - but, and this is important, you will burn more calories by doing so. This is true with both bicycling and backpacking - and it has been tested in competitive cycling. So, get your pack weight as low as possible. You will lose weight while having a better backpacking experience.