> So what would be the proper way to describe an ethnic group that, when examined purely statistically, really has performed well above average in mathamatics?

Exactly the way you said it. "Statistically speaking, controlling (or not controlling) for relevant factors foo, bar, and baz, group X outperforms other groups such Y, and Z in tests of skill A". Which isn't the same thing as "X is good at A". "X is good at A" is a meaningless statement. It's devoid of context and prone to overgeneralization. It's not useful for anything.