But since the maximum power you can output is mostly a question of "will", you would quickly switch from comparing absolute power to instead look at W/bmp, i.e. to try to normalize your tests into a unit basically representing efficiency. And it's more interesting what power you can produce while in your comfort zone - a power you can maintain.

If I ride the same route two days in a row and I go 2 minutes faster one day - was that because I was more motivated, or because the wind conditions was better or I was better seated or I was more rested? Or some other thirteen random differences?

If I'm two minutes slower but at the same bmp, then I am at least not slacking. So maybe the body was tired. Or the wind conditions was worse. Or I was stupid and decided to sit straight up.

The repeatability of our body is nowhere near that of a machine.

But playing with a power meter you can tuck in your arms and directly look at the power meter. Or try the same climb multiple times - sitting or standing. Or switching gear/rpm and see what happens with the power reading.

In the end, I don't think you would be able to test on multiple bikes/frames. I don't think you get enough repeatability.

Different seating positions can in some situations be covered with sadle settings, how long forward the handlebar is etc. So you can get similar seating positions with both a small and tall frame. It isn't uncommon that people have multiple bikes with different size - for example a shorter bike allows you to be closer to the person in front. But is also more nervous.

I think you can gain more from reducing your wind drag factor, than you can from finding the "perfect" position to sit on the bike.