I tried making this work early last year as part of Unladen Swallow, and even though I got it working and it does speed up certain builtin calls, it didn't move overall application performance at all. I believe this was due to cache effects, branch mispredicts or something else. Based on that experience, I don't believe this is really worth pursuing on its own.

That said, something like this is included in Unladen Swallow, which uses it to make direct calls to certain C functions. I'm going to close this accordingly.