I am hesitant to recommend using a test that was last re-normed in the early 70s. I know there are proponents of it for extremely high-IQ persons, but I lean against it. Nancy Robinson (of the Robinson Center at the University of Washington) wrote an opinion for (I think) Roeper Review some years ago advocating for the use of current norms, rather than going back to the SBLM, and she has/had extensive experience with PG children. In addition, the touted higher range on the SBLM is not really applicable to a child of your daughter's age, as it relies on comparisons of younger children to the average 16 year old. I am pretty sure she well exceeds the average 16 year old in at least a few areas!
My thought on the lower scores was that it was not clear whether memory or fine motor affected them. It sounds like fine motor is not an issue. Memory could be, as working memory is not the same as long-term memory or photographic memory. The kind of working memory tapped on this instrument is largely sequential, which can be contrary to the memory process of someone with (near) photographic memory, which tends to be simultaneous. Often, it is spatial in nature. When you describe her gross motor delays (of whatever magnitude), I wonder if those are related to relative weaknesses in sequential memory, as that is what gross motor actions require--learning a sequence of motions.
And yes, working memory is related to sustained attention, as it describes how much data one can hold and process at once (or, alternatively, how long one can hold this data for processing).
I don't think the high PS hurts her WM, but it does mean it's not as important, as she probably is able to finish processing incoming stimuli so quickly that the strength or weakness of her working memory has a negligible effect on the outcomes.
