There are problems with every method of differentiation. I'm thinking about underprivileged gifted kids who might not have the exposure to things 5+ grades ahead, though they may have the intellectual power to comprehend it. Then there are kids who are way out there in just 1-2 areas, but more average in others. There just aren't any easy outs when it comes to summarizing a person's intellectual capacity.

My preference is to look at a number of things, and never pigeonhole a kid based upon a single test.