She sounds a lot like my daughter. She was also a very early reader and extremely poor speller. We had some individual achievement testing done at age 8 and she actually had a spread of 10 grade levels from her strongest area to spelling (her weakest). LOL We have had great progress with Sequential spelling (we've finished level 5 at age 11), but didn't start it until age 9. Honestly, any spelling work we did before age 9.5 or so didn't seem to stick at all. We did Spelling Power and she would miss about 40-50% of the words no matter what practice she did with the words, and given tests over the same words would result in missing different words each time. We did a lot of other things that I think have helped tremendously over time.
We played a lot with words. Rhyming games were a big car game for several years. We studied word roots with English from the roots up and vocabulary books from www.rfwp.com
starting with Caesar's English 1. After Caesar's English the series changes to Word within the Word and it includes spelling. These aren't your typical spelling words, but words like euphemism, pellucid, hemiplegia, etc. Knowing the roots taught her to put together big words and successfully spelling these types of words boosted her confidence tremendously. We made it fun and celebrated any correct words. Word list 1 had her spelling 20-25% correctly, now at list 29 she's at >95% correct.
Typing stories, emailing friends, and participating on online forums contributed to improvement as well. Spell check helped her notice her spelling as she typed. Writing to friends and on forums made spelling important to her.
Because of her spelling difficulties we really focused on playing with language. She has subsequently developed a love of language which has also contributed to improvement in spelling. She now studies Latin and Spanish and will begin linguistics in the fall. Her spelling at age 11.5 is well above grade level and commensurate with her other abilities now. She'll never be a spelling bee winner (unless under very lucky circumstances she gets all words with roots she's studied. LOL) but that is okay since she has no interest in that anyway.
My husband cannot spell to save his life. In fact I think he has a special talent for misspelling words in ways that spell check doesn't catch. We wanted our daughter to avoid some of his difficulties. However, focusing on language more than spelling seems to have been the key for our dd. Improvement wouldn't come until spelling was important to her, and that was after she really got into writing. In my opinion work on spelling before age 9 was a total waste of time for her. If I had it to do again, I'd do phonics (which we did) and play with words and study word roots until 9 or so. Once she was ready to learn, she improved quickly over about an 18 month to 2 year period.