It would be interesting if you could give some arguments for your positions. Why is STM not the way? Why, if IronPython is as good as you say it is, doesn't it see greater adoption or why don't other implementations use its strategies for removing the GIL? Wikipedia says that IronPython scores worse on PyStone benchmarks compared to CPython, and it's likely that this is a consequence of IronPython's fine-grained locking which is required in the absence of a GIL.

As for interfacing with C, like I said, Cython really makes this a lot easier than the approaches you mention. You mention IronPython as not having a GIL, but then IronPython doesn't allow easy interfacing with C code, e.g., it's not compatible with numpy ...