I would like to use a threadless stem on my threaded road fork, and I need to get the bars up pretty high. Who makes the tallest threaded-to-threadless converter? I would rather not put a stem riser on top of the threaded-to-threadless converter, since it seems that would add a flex point and add up to possibly unnecessary extra weight with the additional clamp mechanism. So a tall threaded-to-threadless adapter would be ideal. Nobody seems to consistently show hight specs on these with the online sellers. This seems to be a common need so I figured somebody on the forum might have already done research that can be shared.

To my knowledge, all threadless stem adapters are designed to have the head pushed down against the headset. However they do have more extension below than the 2-1/2" minimum needed, so there's potential to raise them somewhat, but think using a high angle stem makes more sense.

i have a profile threaded to threadless adapter
and it has a clamp area just big enough for the stem
but the quill has a couple inches of height adjustment

and
the stem in no way need to be against the headset
as the headset is the threaded type
the adapter basically turns a clamp on stem into a quill stem

and
if two inches of quill extension
plus a high rise stem
doesnt get your bars high enough
then i suspect you have the wrong size of bike
or a bike that is otherwise ill fitting
to the point of being impractical for you to ride

The one Velo Orange offers is quite tall. I have one on my commuter MTB so I can't measure it right now, but it's considerably longer than a Profile I have on another bike.

VO Threadless Stem Adaptor - Threadless Stems - Stems - Components

Soma Fabrications High Rider -- Soma High Rider Stem Adapter (Threaded steerers)

If you need something taller than this, you've got serious problems.

Super. I had not found the Soma adapter in my own searching. And the serious problems? Yes. Age, combined with the refusal to accept limitations on my desire to keep doing what I want to do.