I'm not clear on how he is going to a district magnet because he tested in, but is not identified as a gifted student. So I guess the answer to that question lies in what he will get out of being officially identified, since it sounds like he is already getting gifted services.
DD has around a 60 point gap between her perceptual reasoning score and processing speed. In the report it says that her slowness during testing was very apparent, in that it took over an hour longer to test her than other kids. The psych recommended a 504 for slow processing speed/ADHD in the report with suggestions for modifications. The ADHD diagnosis was a previous one. If a child "just" has slow processing speed, I'm not sure whether they could qualify for a 504 or not. Aspergers disorder should qualify. However, if he doesn't really have that, it doesn't make sense to have a label and a 504 for it.
DS's took the WPPSI in preschool and the WISC a couple years later. Block Design went up, presumably as his fine motor skills increased. It was still a lot lower than matrix reasoning or picture concepts though. His overall IQ went up a lot between age 3.5 and 6, and I suspect if he was tested again now at age 7, it would be higher than at age 6. It just happens with kids who develop differently, I think. DS doesn't have Aspergers but has some of the traits, and they have diminished over time. He could very easily have been misdiagnosed with it a few years ago.
