I found this on the GoG forum: https://www.gog.com/forum/descent_series/way_to_increase_mouse_sensitivity_past_max_d1_2
According to the last post, Descent has a fixed turn rate. Mouse acceleration only reduces the time until that turn rate is reached (which is almost immediately anyway). Hence you see now real difference between sensitivity 100 and 1000.
If that’s really the case, then it’s an issue with the game itself, and can’t be fixed with Dosbox mouse settings. 😕
I’d have to test it on a real pc though for comparison.