DonS said:It's not quite the same thing.
With a gravity-fed aux tank above the main tank, the main tank is, in effect, always "pressurized" (presuming that there is no closed valve between the two tanks).
A return line that dumps fuel into the "empty" space at the top of a tank is a bit different from one that tries to force fuel into the middle of a tank. The latter has to overcome the pressure that exists below the surface of the fluid. Whether that actually makes a real-world difference in this application...
The kit I have has a "float" valve. I can leave my cap off and it won't overfill the tank