i cant tell you for sure how much of a difference there will be... more light = happier plants = bigger buds = heavier yields... so i would use the light anyways... if you are asking what the yield difference is... assuming you get a 1 gram per watt yield... it would be 400 more grams, or about 15.75 ounces, with the 400w light used... either way it will be worth using the light as long as you can keep heat under control...
sampson