these lamps you have now, are they retrofitted in the canopy? if so, you can pick up another 2 x 54 watt t-5 retro, or even a 4 lamp retro. it's not watts, it's efficiency that you are after.
just so im understanding correctly, the lamps are 160 watts each, and you have 4 of them, or the total wattage is 160?