Matt: Yeah, my softie tank is T5s zoas/palys seem to grow nice whatever, however when I got some blue ones and orange ones, for kicks and giggles I tossed them in the bottom (on a separate rock for easy removal) of my 18" tank with 250w 12k bulbs over it, and they seem to have colors that are just as good, no bleaching out, anything.
I believe the largest issue if you go from some tank with PC lighting to very strong MH lighting (10k) you might get some adverse affects. I'm not really sure how much light a 20k puts out, but I do know that is significantly less than a 10k bulb...