It's easy to speculate about kids- I'm an adult
There is no question in my mind because I'm an adult and I have SPD. I am also a Mom to two boys who have it. My older son has High Functioning Autism and my younger son is neurotypical and I believe could easily be misdiagnosed with ADHD. I do not have an ASD or ADHD- just SPD. I can smell things no one else smells taste things no one else tastes, see things others don't see, and certain tactile sensations bother me. (The aforementioned is not a catalog of hallucinations by the way! LOL)
Let me give a couple of examples: There is a device that emits a high frequency sound that is beyond the range of human hearing that is meant to scare off birds (it doesn't work in case you're curious). They used to have them at a hotel where my husband worked and I made him turn them off because the shrillness drove me crazy. Another example: I can't stand fluorescent lights because I can see the flickering as the spark jumps from one gas molecule to another. The motion gives me headaches and makes me irritable after a while. I can't work in offices and I have to limit shopping trips. I have even seen the number flash on the movie screen that they use to match up reels in the projection room and no one else saw it! I also have the typical aversions to loud noises, overstimulation, noxious smells, etc. But I thought those bore specific mention. :)
The reason I single myself out is because I *know* what I am experiencing first-hand. I am not trying to interpret a child's experience through his behavior which I know can be perplexing and attributable to several factors. That said, I also know that the root of many (if not most) of my autistic son's behaviors are sensory. Fortunately I am good at coping with my "issues" and they don't really slow me down any. It is my hope that someday this will be the case for my children as well. But they will not likely get there without proper interventions since they are more afflicted than I.