> On the whole I think the arguments to make a module are very strong. So I think that is the appropriate way forward.

If you take this route, please don't write it directly against the CPython C-API (as you would for a CPython stdlib module). It is much easier to get something working, correct, testable and feature-rich with Cython. Once you have that, put it on PyPI to give it more testing. If you then decide to come back and provide a patch to get it integrated into CPython's zlib module, it's relatively easy to manually translate the Cython code to equivalent C-API code. But by that time, your code will be known to work, will have a test suite, and we can run real benchmarks on it before investing the time into spelling it out in C.