Positioning cannot be better as it is all run through the same code.
Again, true 3D positioning cannot be accurately done using a stereo source. It requires a mono source.
Whether Miles is merging the stereo source down to a single channel or completely by-passing 3D placement is the only question I have not been able to find the answer to.
If you are applying another filter/CODEC (i.e. DTS, Dolby...), outside of the game, then all bets are off.
By the way, this is not a game limitation. DirectSound/Miles requires a mono sound source for 3D positioning to be done.