If we are talking about Manhattan consider the screening that has already occurred in order for you to be working at a high level in NYC, so if you raised your gifted child there, then chances are you have plenty of gifted families around who are going through the same experiences.

Gifted grouping is necessary because:
-regular classes are too easy,
-the required school hours take up so much of the childhood daytime, the regular classes may waste the gifted child's time,
-the gifted people will eventually work together, somehow, someway and it would be great to learn if it is possible to foster any consensus ever among geniuses,
-gifted children, especially boys, intentionally check out when they are totally bored (the school cannot possibly even know their ability in person (just on the standardized tests) because they are not showing it when they are bored),
-if you are not engaging and educating the smartest kids, is it really a school? an education?,
-seeing the people at the top of the average range get straight As sets those poor children up for a huge shock. Think: You know when the college professor states clearly, " I am only teaching to the top 5% of this classroom." or "Only 1 out of every 3 of you will be here next semester / next year." or fill in your own quote. Those professors are letting you know that they know that somehow admissions let in X amount, but the school knows from historical data that only Y amount of those admitted actually have the chops to do that work at that level consistently. Think: How many levels of screening did you go through (whether you realized it or not) to be included in that group that is sitting in that room with you.
Those are my only comments for now. Hope it helps.