Very impressive exercise but there are a couple of other things that need to be taken into consideration -
1) Are you certain that there is not a recess in the body below the wing that may allow for a different wing-chord than your model had? I've not seen a GT90 with the wing deployed.
2) Relative wind and angle of attack - it may not provide downforce from inverse lift like most wings, but it is a spoiler which may provide downforce as well as aerodynamic braking.
Frankly, I think #2 may have been their goal.
It would be interesting to see if you could put your model in a wind tunnel to determine the areodynamic effect.