Yeah, I was wondering whether the case of Alan applied to hiragana and katakana, or only to kanji. I would imagine that since they are all syllabaries (the reader doesn’t have to do phonemic analysis of each syllable, e.g., distinguish between cat and bat, and only match the symbol with a single syllable), that there shouldn’t be much difference between them, but I don’t really know.
My understanding is that dyslexia also impacts the ability to receive and understand spoken language, but in subtler and different ways than it does reading. If that’s the case, differences in the writing systems shouldn’t matter in this aspect.
Complicating the question is that diagnosis rates may significantly affect the reported incidence of dyslexia in different cultures. Here in North America, we are much more likely to identify the disorder in patients than in other places. Also, to the extent that dyslexia is hereditary, there may be different rates of incidence among different ethnic groups. So simply comparing reported rates of incidence probably isn’t helpful.