In the mid-'70's, Chrysler did some extensive testing between the 727 and 904 transmissions. I believe they were in Stock and/or SS vehicles. If both transmissions are equally prepared and have torque converters that perform the same, the 904 was found to be .10 - .15 faster.

Engine and vehicle weight differences can affect the specific numbers. A heavier, lower HP vehicle would probably see a little more benefit as compared to a lighter, higher HP vehicle.