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Py_SETREF() is used not only for fixing possible bugs. It can make the correct code shorter and cleaner. For example Stephan appreciated this in df78978dacab. In most case the difference between Py_DECREF and Py_XDECREF looks negligible.
Most of the SETREFs in the next() functions in the itertools module replace Py_CLEAR and therefore don't add additional overhead (but may fix possible bugs). The only Py_DECREF were replaced in tee_next() and accumulate_next(). But they are used not in tight loop. Py_SETREF in tee_next() is executed only for every 57th call. It unlikely causes any measurable degradation. Py_SETREF in accumulate_next() is used together with calling PyIter_Next and PyNumber_Add() or PyObject_CallFunctionObjArgs() which takes much more time.
In any case it would be nice to see any measurements that show a degradation.