As you have mentioned & the magazines from back in the day also mentioned, the large object on the hatch is exact that: a "sunshade" not a actual functional spoiler (even Toyota admits it's really a sunshade). The magazines did mention something about the air coming off of the back of the spoiler creating some air turbulance, but it's not enough to screw w/ the handling of the car. It's nothing to be horribly excited about, the car handles just fine.
The only major big thing that you have to worry about, is for those who have installed a 82 - 84 hatch wing/sunshade on to their hatches, but did not install the metal center support rod that goes through the glass. If you're going fast enough the middle of the spoiler will start to flex & move around a bit. If it moves enough, it WILL shatter & break off, even though it's pretty thick fiberglass. This has been documented several times on the forum, the most recent one, was someone in Australia (Norbie).
The only major big thing that you have to worry about, is for those who have installed a 82 - 84 hatch wing/sunshade on to their hatches, but did not install the metal center support rod that goes through the glass. If you're going fast enough the middle of the spoiler will start to flex & move around a bit. If it moves enough, it WILL shatter & break off, even though it's pretty thick fiberglass. This has been documented several times on the forum, the most recent one, was someone in Australia (Norbie).