It'd be good to get more light, 15-18sq feet is a bit much for a 400. At 50 watts per sq ft a 400 should cover 8-9 sq ft, and the foot print is usually good for 3X3 depending on reflector. I'm guessing the dimensions are 5 or 6 ft X 3 ft? If that's the case another 400 would fit it fine. Only problem is heat buildup, 2X lamps = 2 X heat. if your venting is good enough should be okay but just keep temps down. I'd suggest a 400mh. You have a hps already and getting a mh will let you start plants under the MH light, then when you flower you can use the HPS in conjunction with the MH.

Or you can set up a perpetual harvest using the mh for veg and hps for flower. You'll be able to have a more flexible system and the balance of the 2 specs should be good.

peace