a 20,000k bulb is stacked up in the blue/violet/UV light frequency. The problem with that is, you're PAR rating (the ability for that light to penetrate the water column) is low because blue light cannot penetrate very well. With a 20k, you will achieve nice color in your coral, but the growth will (should) be slow.
I started off in the opposite spectrum of the MH lighting scheme as you did. I started w/ 3 250w 6500k Iwasakis. There are a very nasty yellow colored bulb, even when supplimented w/ 2 96w actinic VHO's. We recently changed to 3 250w Ushhio 10k's (w/ the actinic VHO's) and are very happy with the color/growth combination of these bulbs. My coral growth rate was AMAZING (with the saki's)!! The star polyps and my SPS frags would grow at insane rates.
I could only keep corals in coloration that didn't require the high end spectrum (ie mostly green corals). All the other peices that were more towards the blue coloration would brown out at the end of the MH's life (when the spectrum would shift even more into the yellow).
If i were in your situation, i would attempt to try to get some of that lower Kelvin Temp color into your aquarium by using 10k VHO's, or maybe putting another halide on the tank. I dont know how you would do this and achieve a nice color blend, but it would be the best way to get the best of both worlds.
