I have collected statistics about using CALL_FUNCTION* opcodes in compliled code during running CPython testsuite. According to it, 99.4% emitted opcodes is the CALL_FUNCTION opcode, and 89% of emitted CALL_FUNCTION opcodes have only positional arguments, and 98% of them have not more than 3 arguments.
That was about calls from Python code. All convenient C API functions (like PyObject_CallFunction and PyObject_CallFunctionObjArgs) used for direct calling in C code use only positional arguments.
Thus I think we need to optimize only cases of calling with small number (0-3) of positional arguments.