Chrysler testing back in the '70's showed about .15 improvement going from 727 to 904. All else being equal. (May vary a bit based on HP, weight, etc.) It could very well be that the 904 converter is not as efficient as the 727 converter. Only other thing might be a mechanical issue in the trans. Fluid pressure difference can also make a little difference and I was told a trans brake valve body may be a bit slower footbraking than a regular valve body. Possibly because of pressure requirements.