I think you're misinterpreting what is commonly said about what happens with thick walled bassoons in the "upper" register. High notes generally are no more difficult to get to speak on thicker walled bassoons, especially the very highest notes. No different than thinner instruments. There tends to be a feeling of more resistance in the tenor register (say, Mozart concerto Bb up through the next octave) with thick walled instruments compared to thinner ones though. This is not to say they're more difficult to play, or don't speak as well, but they "push back" at the player in a different way. I like to describe the thin walled instruments as having a light and floaty quality in that register, whereas the heavier walled instruments have a heftier raw-power quality.
You would maybe make your reeds differently and use a different bocal on thin vs. thick instruments (although not necessarily so) depending on your taste.
There are other differences in how the instruments respond as well, and in how the intonation is balanced, how much "oomph" the bottom register has, stability of the middle register. Etc. If you have an opportunity to play several instruments next to each other at a conference like the IDRS, MQVC or TMEA, it can be pretty eye opening how different makes and models feel and sound.
M.M.A., D.M.A. University of Illinois at Urbana/Champaign: B.Mus. Lawrence University
Bassoon professor at University of Wisconsin Eau-Claire
Maker of the Little-Jake electric bassoon pickup and Weasel bassoon reeds