Qsound itself is a stereo expansion effect, it is designed to create the illusion of the speakers being further apart than they really are.
The Qsound chip in the CPS2 is a wavetable chip with this effect built in. The improved emulation added the stereo expansion effect, previously only the wavetable part was working. The chip had a few other effects that are now emulated also.
First MAME version with improved HLE emulation is 0.201. And vgmplay version 0.40.9. right now it is fully working in MAME, though an "improvement" in the future would be low level emulation, that is emulating the DSP itself. Currently LLE has some issues with timing in MAME.
As for the specs, I think it's easier just to read the source code. I have a document for programmers
here though.