Its not really the amount of watts that was hurting the anemone. It was the one of two things: photoshock or heat. Photoshock happens when you upgrade your lighting too fast, it can seriously harm an anemone if you change the lighting too quickly. Basically these photosynthetic creatures build up sheilding to the intense light.... when you go from something like PC lights to Halides and start right away with a full 10 hour day of halides lighting... you can kill them.
Always break them into the light, by doing something such as starting a new light at one hour a day, then add a half hour every other day until you are at a full schedule.... or something like this. This will help them get used to it, and they will appreciate the new lights.
The biggest challenge with a 150 halide and a 12g would be the heat, but that is something you can monitor.