I had switched over to the AB 10K (which is actually 13K) supplemented with actinic VHO and liked the look better than the 20Ks. It appeared to me to make my corals look more natural in their color. Also noticedt that when I switched from magnetic to electric ballast my 10Ks were more blue/white than they were before. I can't say anything about growth since it wasn't that long of a period before tearing the system down, but usually growth is better under brighter light (par rating) which is what you usually get with a lower K rating.
Just my .02 worth.