uh.. you definitely can not have clams with a 250w at 15 feet depth.. but man, call me over when you get that cube set up!!!
If you mean 15" then a 250 watt is going to blast the holy heck out of your tank floor with light. You can grow SPS and clams on your sandbed, no need to raise anything off of the sand bed, LOL.
I think the key is (as mentioned above), if you're finding you have too much light, raise your light fixture, not enough, lower it. From 15" to 18" either light fixture should work once you find the sweet spot for where to hang the light.