The absolute, most important thing that you need to consider with the design of a spoiler is its looks, because after everything is said and done, that is its most import design purpose. The stealth looking different from the 300GT for example... I mean they look different but are bot two different shells of a mitsubisi car, kinda iranic that is spoiler came from this car with this discussion. I think there's too much credit given for the wrong thing. I mean air flow can be as important as it can be tricky, but it kinda boils down to image despite how useful one can be, eom is not some sort of vacination to rice.
If you want to consider the usefulnes, remember to direct the air down at the rear of the VX. IT's about the air-seperation at the tail: a wide seperation area means you are sort of pulling or "dragging" around this big bubble behind you.. picture a golf ball, that's why it's not smooth. There was a good discussion about roof racks and tires if you want to look into it, should relate. Good luck with the install, I can't wait to see the pics..