I think it would be good to extract these two lines into their own function. Maybe it's possible to create some ED_slider_op_factor_get(op) along those lines? Disclaimer: I haven't actually compiled/tested this ;-)

// Document assumptions here about `op` having `tGraphSliderOp` customdata.
float ED_slider_op_factor_get_and_remember(wmOperator *op)
{
  tGraphSliderOp *gso = op->customdata;
  const float factor = ED_slider_factor_get(gso->slider);
  RNA_property_float_set(op->ptr, gso->factor_prop, factor);
  return factor;
}

That way it'll be easier to make future code do the right thing. It could also be worth documenting the existance of this function at ED_slider_factor_get().