Curiously, he doesn't seem to present any numbers on the outcome for the gifted children. All he says is that it is better for the other children if they learn together with the gifted children.

I think parents of gifted children are first concerned with how well their own children do, not the others.

I question that even "learning together" as in school is the most effective way to learn. Some homeschoolers achieve stellar results (see Polgar sisters), indicating that there may be better ways.