Not much experience but i'v noticed that if you experiment with different amounts of light you might see some unexpected alge growing from your live rock.I'v seen many types of Caulerpas,Halimeda and Sargasum grow form my live rock.
Dennis photosynthesis is most efficient at about 6400K (I think). If you want to use the 40 watt compacts I would go with one 40 watt but at a higher kelvin rating like 6500 - 7100K range. I currently use the 6700 compact bulbs they sell at home depot and grow macro algae underneath them without a problem.