Not so fast.

I'm not in favor of (c) co_doc either any more (for the reasons you state). I would go for (b), a CO_DOCSTRING flag plus co_consts[0]. I expect that co_consts sharing to be a very minor benefit, but you could easily count this with another small change to the count script.

Moving the docstring to the surrounding object would not make much of a difference time- or speed-wise but I think it's the wrong thing to do since it dissociates the docstring from the function.

Nested function creation could perhaps become a fraction faster if we didn't copy the docstring into the function object, leaving it func_doc NULL, making func.__doc__ a property that falls back on co_consts[0] if the flag is set.

I expect lazy docstrings to be in the distant future (I experimented quite a bit with different marshal formats to support this and it wasn't easy at all) but I don't want to exclude it.