Fullom, mastering does not equal "making it as loud as possible". That's what the gain/volume knob is for. CDs that are mastered 'quieter' will have more dynamic range (difference between loud and quiet sections) and will therefore tend to sound more 'alive' and more interesting when listening through a hi-fi setup. CDs limited to buggery (i.e. the loud ones) tend to be fatiguing to listen to for any length of time.
IMO the best mastering I've heard of late on a psy-trance release is the most recent Cosmosis album.