All the watts to consider is really not the only consideration. The distance they are from the surface or the interval will also play a large roll on the amount of all those watts that gets into the tank and to the coral. If you have to keep the 400 watters way up high to prevent heat issues, then they may not even be as effective as a 250 mounted closer to the water with a good mirror reflector. So long as you have the mix right on the wave length and enough total watts getting into the water, the clams and coral will do great under 250's with VHO's.