I am not familiar with the WJ Cog but imagine that it probably is not so very different from the WISC. As far as disgraphia, the only subtest affected is coding (1 of 2 in the processing speed section). As for dyslexia, I don't recall that any reading is required. Of course, there could be an indirect effect to the extent that dyslexia has impacted the acquisition of knowledge/skills, which feed into the measure of crystallized knowledge (verbal comprehension index on WISC).
In my opinion, any of these IQ tests may either overestimate or underestimate "true" intellectual ability on any given day. Different IQ tests may also not correlate exactly as they do not measure exactly the same kinds of ability to the same proportion, some versions are more in need of renorming than others, etc. I believe that they are more useful as an approximation of a general range.
Furthermore, your DS' actual functioning in real life is probably more indicative of true ability and ultimately is what actually counts more than an IQ number that may or may not be given weigh by the people in authority.
I always find the "gifted traits" a bit suspect. Our district acknowledges that many kids have these traits to some extent but tried to distinguish it by focusing on the frequencies these traits are demonstrated.
