I'm not familiar with the Stanford Achievment Tests, but things to watch out for are whether or not the test is timed vs untamed, whether or not it requires a handwritten response or an oral response, and whether or not the prompt given is verbal or the child is required to read the question. If your child has a disability impacting reading/writing/processing speed any of those areas might cause an achievement test to not accurately reflect your child's knowledge.
And yes, gifted children with disabilities absolutely do participate in gifted programming - it's really important for most gifted kids to *not* focus solely on remediating their area of weakness but instead place them where they need to be intellectually and give them support in their challenges (remediation and accommodations). That said, we did have difficulty getting our 2e ds identified early on in elementary school because although he had qualifying ability scores, his achievement scores were inaccurate in some areas due to his disabilities prior to his diagnosis (they were still inaccurate after his diagnosis - but we knew *why* they were so low then and were able to get him accommodations that allowed him to show his knowledge during testing and in the classroom). We still focused more on helping him in his areas of challenge while he was in early - mid elementary school, which I think is what we had to do in order to help him get to the place he is at now, where he can really take advantage of his strengths in middle school (he's now subject-accelerated in an academically challenging school).
I'd recommend seeking out a full educational evaluation from a private neuropsychologist, or at the very least educational testing/screening for dyslexia. Both of my daughters struggled with reading (one was more obvious than the other) - and they both had that "flattening out" where their achievement started to lag in reading - but for very different reasons, and neither is dyslexic (one had an undetected vision challenge, the other has a challenge with associative memory).
Good luck finding answers as you go forward -
polarbear
