SPDY wasn't something I had heard of before I read this article but it looks like it's a binary implementation and enhancement to HTTP which has been adopted as the basis for the HTTP 2.0 spec. With that in mind, this implementation of the SPDY protocol in Objective-C is worth a look. I would imagine that to find this useful right now you would need to be solving some fairly big networking problems (like Twitter, for example) but that doesn't make it any less of an interesting read.
Graham Lee with a deep dive into the Objective-C runtime looking at how the Foundation implementation of KVO works, then going on to reimplement the Observer Pattern from first principles before ending with going all Ruby on us to implement method_missing in Objective-C. If there wasn't enough low level content in the LLVM talks mentioned above then get your teeth into this as well.
I've been meaning to learn more about Core Bluetooth for a while now but this article by Steven Daniel finally made me actually take a look at it in more depth than "Oh, I know that thing exists". I was surprised how trivial it was to manage connections and talk with bluetooth devices using this framework. Worth a look if you haven't had a chance to investigate Core Bluetooth yet.