I don't think it's as much UV as it is some peak in the 420 nm spectrum, that's what many generally believe to be true. If you look at the peak for most LED units--including the higher end ones, they peak at around 430-450nm. It's believed that the best spectrum for coral is in the 420nm wavelength. Actinics generally peak in the 420nm, so if you supp your LED's with T5 actinics, that should be the best combo based on present belief.
Yeah, people keep saying "we don't know" people on internet forums may not know, but there is actually a lot of research. So, there is certainly things we don't know, but "we know" more than most people seem to think.
Here is some good info on uv "Playing with Poison"
http://www.advancedaquarist.com/2004/8/aafeature
Danna also has written probably around 20 or so articles, summarizing scientific research, as well as his own research, on coral coloration and color pigmentation. I am not going to link all of them, but here is the first of one of his series. You can search further if interested.
http://www.advancedaquarist.com/2009/1/aafeature1
This is a topic that really has been surprisingly well studied, it just doesn't seem to make it to the hobby. For whatever reason, hobbyists seem determined to reinvent the wheel sort of speak. Perhaps much of it is just too technical. The jist of it though is coral coloration is a response to high intensity blue light, generally in the 400-500ish nm range. Also note, pigments can either reflect light, or fluoresce i.e. absorb light and emit light of a different color. If you reflect UV light, the reflected light will be invisible, as we humans can not see uv. All pigments found so far to respond specifically to UV (i.e MMAs) are reflective pigments and therefore not visible. There is one slight exception, realized within the last few years, this is Green Fluorescent Protein GFP. GFP, takes high energy light and reduces the energy by absorbing some energy and re-transmitting at a different wavelength. However, it isn't specific to UV, it responds just as well to high intensity blue light.