A 400wMH should be enough to produce around 80% of the maximum yield that you can get from a single plant (depending on temperature, mylar, bugs, deficiencies etc).
Try putting the HPS at the side of the closet, for sidelighting. Youll get a few more grams because more light is getting to the lower buds. If you do this, make sure you turn it round 180 degrees every other day.