I debugged it, event with big mouse sensitivity (4000) does not turn faster (tested with descent 1). It stops turning faster after reaching some value. I checked emulator in core and there is no limit on it, so it really looks like game has turn speed cap.

>using dxx-rebirth as this is a no brainer, it has been fixed there.
You are right, but this is not the original game. Dxx-rebirth is (taken from web site) :

“DXX-Rebirth is a source port of the Descent and Descent 2 Engines for Windows”

It means, it is coded from scratch and has nothing to do with original dos game.