From talking with Edge 305 owners (and sometimes comparing their data to my own Polar and ciclo), it appears that after running the elevation data through the Garmin Training Center software, cumulative elevation gain/loss reports compares consistently with other barometric altimeters such as VDO, Polar, Ciclo, etc.

Forerunner 305 data which only uses GPS for elevation can report the elevation correctly, but not the cumulative gain/loss. I've had it report as much as 3x the actual.