Welcome tjd! Of course, you'll have to remember where you're asking your question... The GT school you've described sounds great, and even the folks at the private school concur it would be better academically. I am jealous that your public GT school has such small class sizes. We would jump for that.

Since the private school sounds relatively inflexible re: accommodating your DD's GT needs, and they have been up front about that, I wouldn't even consider it.