I agree that as long as there is full reading comprehension, there doesn't seem to be anything to worry about!
I can't remember if I shared this here or somewhere else, but I had the same concern about my older DS this past year. At 5y9m, he flew through the first Harry Potter book in 3 1/2 hours. I was extremely skeptical, esp since it took me at least that long and I'm an ol' attorney who can fly through the books. Nonetheless, I gave him a Harry Potter middle school reading comp. quiz, and he aced it. He then proceeded to argue a point with me by rapidly opening the book to where the passage was. That tells me they CAN comprehend that quickly! (He also tends to read an entire Percy Jackson novel in an evening.)
Since my DS is still little, his writing hugely lags behind his reading. However, when my stepson turned 16, we discovered that he has a learning process disability where his brain isn't able to transmit his thoughts to his hand very well. (There's a book, but I can't find it here on our bookshelves!) He is MG and has also always been a voracious and extremely rapid reader. He now gets accommodations for his writing assignments at his Waldorf high school. He tried a voice-typing program, but I don't think that worked well for him. (He never uses it when he's with us.) It is considered a disability in terms of special accommodations for the SAT and other tests. I'm sure your DS is probably too young for that type of concern yet, but it isn't unheard of for the reading to be high but writing to be low.
