I meant with a threadless fork as well. I imagine most of the benefit comes from the direct interface and the tubular nature of the stem extension vs. the tube crammed in another tube aspect of the quill stem. They move around more than some people think. I haven't used a quill-to-threadless adapter for very long, but it certainly didn't feel super flexy like, for instance, a 3ttt Synthesis, which is the stem on my road bike. Just not as stiff as a threadless fork and stem.

The Technomic isn't very stiff, sure is tall though, and that's why it's most useful. Dynamics are shorter, stiffer.

The stiffest quill stems in my experience are the lovely (or hideous) chrome plated steel stems from the 90s. I saw this as a drawback at the time, but you might consider it a feature.