An example should make my concern clearer. str(signature(int)) is '()' because int (and all other builtins) has not been converted whereas the Idle calltip is
  int(x=0) -> integer
  int(x, base=10) -> integer
I do not want to simply add a line with '()' to the top of the calltip, so a little thought is needed to improve the calltip for the rare cases above without degrading them for existing, more common cases.