im definitely no expert in the field, but am doing postgrad research into similarities between PKU and ADHD, and i would suggest that maybe the high Phe levels may be contributing… while 300 is considered safe for those with PKU, its not optimal. and its definitely not uncommon for people to have behavioural/cognitive symptoms at these levels.
its always the case that the individual will provide more information than numbers ever will. numbers are always somewhat arbitrary, whereas behaviour speaks for itself.
also to keep in mind is that asperger's is a diagnosis to be always taken with a grain of salt. so i believe youre definitely onto something by questioning the relevance of high Phe levels.