Without experience (with H. mag under T5 that is), I can only speculate. But personally, while I think the intensity might be OK, it's something I wouldn't try. I've found that having the "spotlight" effect of a halide directly overhead the anemone really is profound how helpful it is to encourage the anemone not to walk.
When I had my ritteri in a tank under two halides, neither of which were directly over the anemone, ie., slightly offset on each side .. if I had trouble with one of the lights firing, the anemone would move to the side with the working light *within 20 to 30 minutes* after the lights should have turned on. It's as if he said "Oh - the sun is over there now. Ok, off I go!"
Just my guess. I like T5's too but I personally would only consider T5 lighting as supplemental for my ritteri tank. Sorry I know it's not what you wanted to hear!
Give it a try and let us know, but like I said, if it was me I'd stick with the halide.
PS. Personally I think the intensity needs tend to be a little overstated. I did keep my H. mag under 175's for the longest time and even now, at >5 years I still only have it under a single 250. I know it can take more, but I feel that it doesn't *need* more. But what it *does* need, I think, is that "spotlight" effect to help keep it rooted in place.
