So I would not be so sure that there was not a significant input of lift from the surrounding environment.
you calculate with stopped turbines, (correct, a stone need a start speed of vertical 134 m/s to reach a delta-high of 914m against gravity...... he need 13,6 sec.)
but with decreasing speed during climb no longer every energie of the turbines is required against air drag, so they provide more energie into the kinetic system....