Harry Potter hadn't been written when I was a child, but I was an early and enthusiastic reader. I first read The Lion, The Witch And The Wardrobe at 7, and the language is a little more dated in that. The themes of HP6 are little darker than they were back in books 5 or 4 so having come this far, I don't see much difference in accessibility, or scariness.
I enjoy reading and rereading, and to me the mark of a good book is that it can be read again and again with changing understanding. My understanding of books has been different at 7, 17, 27 and 37. Infact the greatest difference in my interpretation of literature has been in viewing the world through the eyes of a parent. Reading books like Roald Dahls' to my 7 year old (delayed reading skills but great comprehension) there is a conflict of my darker adult understanding of the world and my recollection of being his age and that lighter, superficial filter of childhood and him understanding them in the same way that I did.
If a child is simply decoding with no comprehension then that's a hollow experience. What is more likely for this kind of motivated independent reader is that they have sufficient understanding to follow the main plot and obvious details which is fine. A richer understanding including the full darkness of the likes of Voldemort's evil comes later. Before that it's factual, Voldemort kills Cedric, Bellatrix kills Sirius, Malfoy is about to kill Dumbledore but Snape does it instead.
Incidentally, I haven't twigged the word play of Knockturn Alley before reading this thread
That's evaded me on numerous re-readings over the last 17 years or so! But that's why the series is so loved, because it is detailed and rich enough to stay fresh again and again 
I've read the first book to DS and his reading ability puts me in control of the pace of reading the series (I'm gradually buying the illustrated versions as they are more accessible to him) but if he was able to pull them off the shelves to read through at his leisure then I wouldn't censor them and hold him back. If only I could process verbally reading 100 pages an hour like I could in my head! (I miss the days when the HP books were published and I'd await the delivery and spend the next few hours devouring them)