I'd absolutely recommend Fahrenheit 451 to a young gifted reader, if for no other reason that because the theme would resonate strongly with them. I can't recall any adult themes offhand. And if she's interested in Bradbury, I could also recommend Heinlein's works, though I haven't read too many of them. Generally speaking, you can't go far wrong on sci-fi for a tween if it was written before the 70s, as the violence and sexuality were a lot more muted back then.
I gave my DD a shot at some Discworld (Pratchett) material when she was 8 and expressing an interest in my bookshelf, but it didn't take, and I expect that has a lot to do with life experience. The things they make fun of are not things children are not participating in... things like government, law enforcement, communications, university staff, etc. I'm not sure I'd recommend them to an 11yo unless they're already interested in social science-y things.
Given that you've put both Pratchett and Adams on the list, I assume satire is an interest, so I'd add two names:
Mark Twain: Not only great satire, but also great use of language. If only we still wrote like this.
Carl Hiassen: Until recently I would absolutely NOT have recommended his works to parents for their younger children, as they are rife with adult themes, but in recent years he has begun writing a separate set of books targeted at the young adult market, with more appropriate themes. I haven't read his young adult stuff personally (though DD has a copy of Hoot by her bed), but I can say that as satire, he's hilarious. From one adult to another, if you like Adams and Pratchett, you should give Hiassen a try.
