I asked about differences in kelvin the other day and apparently 10-14k provides the best growth. I have a 20k on mine (as well as antics). I plan on replacing it with a 14k after its used up.
Mines about 7 inches above the water surface right now, and I have no heat issues. My tank runs between 80 and 82 (some might say thats too hot, but actual ocean temperatures are around that range and everything is absolutely thriving), which is the same temperature it ran at before I added the halide. If you have the tall canopy and a bunch of fans, you should be ok.
Another alternative if you are still having heat issues is to put some cooling fans over the sump. You'll get more air exposure down there, and if you have sufficient flow, all the hot water will flow through and cool down before heading back into the dt.