Hi Phil; I don't know for sure, which is why I would love to see how it would change

I've keep giganteae 12" under a 175 MH, and they still had long tentacles, but the purple color was more intense and covered more of the tentacles. Additionally there was some flourescent green pigmentation on the oral disk (particularly the mouth).
In the case of BTA's, the lower the light they are kept under, the longer their tentacles get. Under bright light, their tentacles are short and compact. Additionally, under low light they tend to lie out in the open, while under bright light they hunker down into cracks and crevasses.
There is also a big difference between PAR and UVR. Under bright PAR, an anemone will respond one way, while under UVR it will respond very differently. There has been research that shows that some pigments in Actinia species block UVR, and that under higher levels of UVR (ie shallower water in the wild, or UVR lighting in captivity) pigmentation darkens and spreads. So whereas UVR lighting in captivity might not benefit an anemone directly, it may cause it to "color up" so that an aquarist has a more vibrantly colored individual. You can see this quite often with SPS corals where their pigmentation darkens or lightens and changes color depending on the light under which they are kept.
Here's an interesting article on Photo acclimatization mechanisms - on corallimorphs versus anemones. I am not aware of any studies on anemones, and certainly nothing specific on S. gigantea.
You could be a world first