They don't necessarily put out less heat. The closer your bulb is to the water, obviously the more heat will be generated at the surface. Actually more heat, more UV, more evaporation, and more salt creep on your reflectors and bulbs. All in all it is always better to raise your lights if you can generate enough light by doing so.
My experience has been as I have owned both LumenArcs and LumenBrights is if you raise LumenArcs above 12 inches, the amount of light is significantly reduced due to the diffuse spread they provide. Not only does the intensity become reduced but the light spills into your living room even more if you keep your canopy open or on your back wall. The LumenBrights are designed to keep the light within the tank and raising them is showing some pretty impressive numbers to say the least. Like I said before - 200-400 on the bottom of a 30 inch deep tank. At 14-16 inches, this just would not be possible with a LumenArc with the bulbs we have tested so far.
You can investigate and decide for yourself here:
http://archive.reefcentral.com/forum...&pagenumber=10
And you can purchase these from several places:
I bought mine from here: Mike is a great guy and will spend as much time as necessary to help you in any way.
http://www.reefspecialty.com/
Here is another place you can find them:
http://www.reefexotics.com/
The debate on dimensions is pretty interesting - I have seen many different dimensions listed - I know my large one's measure 19.5x19.5x8.5 tall. The minis should measure 16x16x6.5 tall - a little bigger than the LAIII minis.
Hopefully that helps.