Heh. I like the 'testing' she did with telling Santa and not telling mom and dad. Sort of like a scientific hypothesis, then testing it out. THAT sounds like a gifted kid to me. Now that reality is starting to intrude as she gets old, she has come up with another theory.
Regarding growing out of it, she might think that if she admits she doesn't believe, then Santa will stop coming... so she is probably looking at more and more outlandish explanations. If you think it is stressing her too much, you should probably just have "the talk" with her about Santa. When it was time, we told our oldest D that Santa is everyone who loves her. And asked her to please not tell the younger sibs if they haven't figured it out yet (some older kids like to be in on the secret with their parents anyway).
I don't think this is anti-gifted (as you stated) in any way.
