With wings like that, as long as you can mount it high, away from the boot/hatch, then the car its mounted on has less effect, especially on a car like a 240Z which slopes away gently from the roof line. If it's mounted too low then the low pressure zone under the wing which reduces lift also creates a low pressure zone above the boot/hatch and has the opposite effect, increasing lift. Also, the further you can get it back helps because this keeps the low pressure zone behind the car rather than above the boot/hatch.
I found this for anyone that might be interested.
http://www.marsperformance.com/html/MARS-3DI-144CM-Carbon-Fiber-Rear-GT-Spoiler-Wing.html
From the website, I'd say you're right, definitely just a cosmetic part so I'll give it a miss.
lol, could just see it on the Swift! Actually I think I've seen something similar before!