99th percentile in achievement testing doesn't really tell you much about cognitive abilities, because achievement is a combination of cognitive, exposure, and effort.

My DD scored in the 99th percentile for reading and math on the Woodcock-Johnson achievement tests. Cognitively, she's in the moderately gifted range overall, with visual-spatial abilities just shy of the highly gifted range cutoff.

As for cognitive testing... have you spoken to your child's school?