Myelin increases the speed transduction of nervous signals and therefore increases speed of thought but not necessarily intelligence...

As for Autism, I suspect that what causes it varies a lot. Like many conditions, I think it may suffer from the inevitable but inavoidable problem of our symptom led diagnostic system which tends to group conditions with similar symptoms into the same group and assume they have the same cause when often this is not the case. IBS (irritable bowel syndrome) is one such as is Bechet's disease and a number of other genetic conditions. Basically, we class Autism as a condition characterised by specific symptoms which could be caused by any one of a hundred or more different causes - gene mutations, drug therapy in the womb, steroids, the list is endless (though I think the evidence is mounting to discount MMR vaccine now - the original evidence was based on 8 subjects, one of whom was Autistic at the end of the study and there was no way to know if the child was not already autistic before the trial and proper diagnosis is difficult at a very young age. Plus the results have yet to be verified by independent sources). Modern medicine is edging slowly towards the idea that the cause is more important than the symptoms and should determine classification more than the symptoms and modern techniques can now find these out more accurately. This method may allow better prevention and maybe even treatment of some conditions such as Autism.

The too many connections thing is a viable theory as to why autists think and see the world differently to others. It also explains thier other symptoms. Its evolution trying something new to see if it works and finding that it doesn't quite work well enough. Its also my theory about Downs as well - Downs patients have a whole extra chromosome of genetic stuff so again new things being tried.