I have talked to many and seen several tks with SPS and even clams under a 70 watt HQI .... Why is it that everyone thinks you have to blast the crap out of your tk. Believe it or not, there is such a thing as too much light.
To have additional lighting to simulate dawn/dusk is gr8, butI'm talking about blasting everything at once ... don't overdo ... It is unnecessary and will cost you a lot of $$$.
I think a 70watt will give you many options, reduce algea outbreaks and allow for the most broad spectrum of inhabitants. 150 watt would work, but I think is borderline too much. Anything more than 150 watts is overkill and a waste of money. MHO