Is moving to 250 (z = 10) passing a sort of threshold of intelligence

First, IQ tests don't go to 250 :-) Generally speaking standard IQ tests have poor resolution in the tails -- they cannot reliably identify whether you have the IQ of, say, 170 or 190. At some point all you can say is something along the lines of "this person is in the top 0.1% of people we have tested" and leave it at that.

Second, "achievement" is a very fuzzy word. People mean very different things by it. And other than by money it's hard to measure.