1. It does sound like the VCI and PRI could be considered low estimates, between her perfectionism and the lack of natural ceilings.

2. In my experience, cancellation draws heavily on visual scanning skills. It's a large field of visual stimuli, with either some or negligible visual structuring (depending on the trial). Fine motor is a fairly small aspect of it (and probably not the major factor in this case, as the other PSI scores, one of which has more fine motor demands, were higher). Sustained attention and visual organization also affect performance. I wonder how much of the lower performance on Cancellation had to do with maintaining a smaller, more structured visual field for scanning and tracking.

3. We have insufficient information to nail down the differential diagnosis, obviously, but FYI, where low processing speed fits into dyslexia is in decoding automaticity. This is not, btw, incompatible with reading extremely quickly, for a child with very high cognition and memory, as one can reach high reading rates using predominantly whole-word/sight reading. In order to detect the lack of decoding automaticity (sometimes called orthographic mapping), one would have to administer timed tests of decoding nonsense words (such as the Decoding Fluency subtest on the KTEA-3), to reduce access to compensatory strategies that involve sight vocabulary. None of the achievement measures you listed included such a test.

I agree that pinning down the role of vision is key right now, but I would keep dyslexia in the discussion, as that might be another factor affecting comprehension (she may be reading purely by sight, which occasionally results in key miscalls in comprehension passages).

And yes, perfectionism is almost certainly in the mix. Whether it is primary or ancillary is another question.

...pronounced like the long vowel and first letter of the alphabet...