You're right that posture changes but that is the default provided by an industry that assumes a rider wanting flats wants a more upright position. This leaves people wanting a lower position little choice but to go for drops. Selling the same frames with different bars is wrong in my view. And this makes a drop/flat comparison rather nugatory.
But it is possible to make the comparison if you use different size frames and stems. So, in my case a 600 top tube plus 120 stem with flat bar is roughly equivalent to a 560 frame plus 90 stem with drops and hands on the hoods so I can remove the posture element and concentrate purely on the way the hands make contact with the bar.
Basically, I don't find hoods comfortable and they make braking very much more awkward.
Interesting, I don't doubt for a second that some people won't like the hoods position, regardless of how it's setup. But I am fascinated by the variance of view on frame size, or at least TT length, required for drops v flats. Partly because I've just converted my Vaya back to flats and wouldn't have minded having the 60cm frame now
I think the fit's ok, TT 585 and drops with 100mm stem to flats with 130mm stem and saddle back by 10mm, so I've gained 40mm on the deal, whereas that would be 55mm with the 60 frame. But I did look into this a bit before trying the switch and there seems to be differing views:-
At one extreme you have the flat bar tourer market and bikes like the Thorns - these make a big variance in TT between flats and drops, very similar to the numbers you mention
In the middle you have things like the Van Nicholas Amazon Rohloff which makes a variance more akin to what I have on the Vaya
Then there're a lot of flat barred road bikes that seem to make little or no variance whatsoever
Finally looking at the 29er market then the variance is around the midpoint as well - for example my new build is specced as 620mm TT and 100mm stem which compares to the Vaya at 585 and 130. My road bike is 570 and 110 by comparison, though it has a bit of saddle to bar drop as well.
I'll see how I get on with the Vaya in this configuration and, if need be, can always get the 60cm size...they do it in Ti