Hi Giftodd,
It sounds like your daughter has some amazing strengths. The reading mistakes that your describe are very common in people with dyslexia - substituting words that have the same initial letter, letter reversals, ability to recognize a word one minute and then not the next, difficulty with rote tasks such as math facts and poor spelling.

That said, the fact that your child is reading at least 5 years ahead (regardless of her IQ) is pretty amazing. My very dyslexic son is in the 8th grade and despite very intensive reading remediation, reads at about the 5th grade level. He is not has bright as your daughter, but very bright none the less.

I think it would be wise to keep an eye on things, but remember that she most likely has had little instruction in phonics/phonemic awareness. This tells me that she was able to break the code on her own. This will impact her spelling. True dyslexics usually cannot do this without explicit instruction. In fact it is their inability to understand and apply the code consistently and independently that is the hallmark of the disability. As you noted, letter directionality does not fully develop until about 2 or 3 grade. She has great rhyming skills - rhyming is something that most dyslexics cannot do. (part of the phonemic awareness deficit). Her fluency is great - most dyslexics struggle with rate and accuracy.

Here is a link to the international dyslexia association - you may want to bookmark it to keep an eye on things http://www.interdys.org/.

Here is a link to a table for reading fluency. Again, you can use it for ongoing monitoring - but she has most likely already achieved top level rates for older age groups. http://www.readingrockets.org/content/pdfs/Hasbrouck-Tindal_chart.pdf

It sounds like your daughter is doing very well. The fact that she is reading FOR MEANING at higher grade levels is very impressive. The fact that she can go beyond the concrete at such a young age shows great promise. I really wouldn't worry about her "relatively" lower performance on certain aspects of decoding and encoding. These are very low level skills (necessary, yes) and she has demonstrated mastery well beyond what is expected at her age.