How real is a tall head tube to ride?

With Paris-Roubaix, everyone's talking about the value of "relaxed"
frame geometry again, specifically the long head tube and the benefits
they bring.

My question is, what ride differences are there b/n a frame with a
longer head tube and a regular road frame that has more spacers inserted
below the stem, thereby achieving a longer virtual head tube?

I've asked this question a number of times elsewhere and no one seemed
to be able to answer it directly.
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