Its not like people have done controlled experiments on the best lighting for S. gigantea. Most people that have been successful have used 250-400 watt metal halides.
I don't know if enough people have tried to keep S. gigantea under T5s for us to have an educated opinion. The hardest part is finding one healthy enough to last the first month or so even with ideal lighting.
I am currently trying a small green gigantea in a 16" tall tank with 4-54 watt T5s. Its only been a week so far. I figure if the lights can bring the purples and blues back to browned out sps, it is probably good enough for a gigantea. The problem is, it will take about 6 months to find out.
In my opinion, I think you have a good shot with 8-54 watt T5s, as long as the anemone isn't more than about 18" below the surface.
Do you already have the anemone, or are you searching?