I don't think the cortisol thing has all that much to do with stress.
I know, I know... cortisol is associated with stress. We all know that. But you have to remember that it was the peak in cortisol upon awakening that they measured differently in Asperger subjects, not overall cortisol levels--those were the same. That peak doesn't just have to do with cortisol, but with the entire hypothalamus-pituitary-adrenal interaction, and at the base of it, with the brain itself, including sensory input and processing.
Also--the study was preliminary, with a small number of subjects, and especially considering that that cortisol peak is absent in many typical people. So this needs more research.