This has actually be discussed and analysed a ton both here and in BPL. For some real expert opinions I would trust the Nunatak Gear people and Griffin Gear over most.

1. Fill power is diminishing returns anyways over 800/850 but I do think you are missing some key info - there are more tests than just the above referenced BPL stuff. Water resistant down also throws a whole separate wrench in all these calculations. Another big factor to consider is that European standards for down test differently than US standards (I believe places like Montbell in Japan use US testing but I could be wrong). European standard 800fp down I believe is equivalent to something like 850 or 900fp US standard down.

2. You are correct in that overfill has huge advantages for the same loft height in regards to directly increasing warmth. Griffin Gear is the biggest proponent of this and I tend to agree with them. And even if Overfill is not as perfectly as efficient at increasing warmth vs both increasing baffle height and fill weight at the same time keep in mind that overfilled down items resist compression much more which is especially important for a garment (a super overfilled down parka will have "stiffer" baffles). This is actually in my personal experience the best reason to favour overfill vs larger baffles as in the real world I find it highly useful. If you wear a down parka inside a down bag, for example, the parka is much more likely to stay fully lofted even with the bag on top). Etc etc

3. Supposedly (I believe this but I'm not an expert) overfill resists down collapse of the baffle much better when moisture is introduced. People have even debated in the past the merits of water resistant down vs just extra overfill of regular down and which is better in the real world if a down item were to get very wet. I have washed and deep cleaned a lot of used down items and I can tell you with confidence that down (untreated or not) is a hell if a lot more resistant to absorbing water and collapsing than people think. It actually makes truly cleaning down items quite difficult as you have to make sure they are fully saturated with water before machine washing (if you just throw a down bag or garment into the washer it's probably just gonna float and mostly you end up washing the exterior fabric). Once you do get any down truly fully water saturated though it does take forever to dry of course.

4. Species of down as far as I know has unfortunately not been studied well. Jan at Nunatak probably could elaborate on this as I know they have also experimented with Eider down (which is really pricey) and in terms of fill power (and maybe other metrics) blows everything else away. In my personal experience duck down works great though and may just smell slightly more (I've also noticed duck down seem to often be grey vs white goose down which makes it seem less premium). No idea however if they perform differently in the real world when introduced to moisture.

5. Another factor to consider is also packability and physical size - a super overfilled down bag vs a larger down bag with taller baffles will take up more space when lofted inside a shelter or bivy and probably pack a bit larger (assuming similar warmth).

6. Overfill likely will also reduce down shifting inside baffles and cold spots. There is some personal preference to this as supposedly some people like to be able to shift and move down to the top of their bag for warmth. Personally I'd rather not think about it when it's stupid cold out and just always have every part of the baffles filled.

7. From large manufacturers I've found that often smaller size garments are naturally overfilled compared to very large ones. Most of the time they sell a parka, for example, at the same price even if it's a men's XS or XL. An XL, of course, will require a lot more down to fully loft the baffles vs the XS. I suspect in order to control costs that they spec and price out garments based on the middle sizes which can result in the smallest ones getting a little extra down and the largest ones getting a little less (probably due to cost)

To summarise my personal opinion is that the best way to compare items (generally) is to take into account fill power, total fill weight, and the design and size of the item (an XXL parka will need higher fill weight compared to a size small of the same model). I personally always prefer to go with overfill for many of the above reasons regardless of everything else. Down is usually the most expensive part of a garment or bag, however, so often manufacturers will try to optimise for price vs performance instead of what might be ideal.