Making it a function might help with the following issues:
- series of comparisons and PyBool_FromLong is less efficient than switch and Py_RETURN_*. But it would add a function call.
- it might be too complex for a macro

Do you think that would help?

As for the signature, I would like this to mirror richcmpfunc and PyObject_RichCompareBool. And returning PyObjexct*, not 0/1, is an important part of reducing boilerplate; in cases where 0/1 would be helpful you can easily work with cmp-style -1/0/1 values before using this to convert them.