As one of the authors of Happy, your post got me very interested. We wrote the paper over a year ago, we used a version of PyPy from back then. It's possible that we get a bigger speedup with a newer PyPy.

It's interesting that you mention the advanced features. I looked at Hippy and the most interesting JIT feature Hippy uses is _virtualizable2_, which virtualizes all function locals and unboxes them. We tried using it ourselves, but it forces each function to have a static list of variables and no dynamic variable accesses (like $$x). It looks like Hippy falls back to the regular implementation for dynamic variable accesses, where the entire list is stored in a dictionary. Now I'm wondering how much this happens in real-world code, we assumed it does happen enough times.

Also, I'm working on posting a publicly-available version of the paper. I'll post a link when I do that.