That looks like a nice way to add support for other libraries or optimize certain operations. But I'm more interested in a comparison between dynamic languages for quickly writing game logic. It's a pattern that seems fairly common (e.g. with Lua or Python bolted onto C/C++) and Godot's implementation of that idea is very unusual. They claim benefits for it, I'm curious if those benefits actually pay off in some quantitative fashion.
Main advantages are tight integration with engine and ease of use. As I wrote here: https://news.ycombinator.com/item?id=15310477 GDScript is not very fast but It's rally an issue. In 3.0 where we will have C# and GDNative it won't be an issue at all.