On a 40b sps tank, a pair of 175/150's would be much better than a single 250. Using quality reflectors (lumenmax/lumenbrights) is the most important factor. Definately not overkill as the best SPS tanks out there usually use "too much" mh light. If you are just starting, a pair of 175 Iwasaki 14k's will give you great growth and color. Heat is really a non-issue if you mount your lights 12"+ above the water. A small fan should help out on those hot days. I have a RK controller to run all my lights, set to turn them off at 85*.