Reading through this thread got me looking for some good wind tunnel pics online, which led me to the TECHART Gallery.
When it comes to high performance automotive mods, I am fairly ignorant, but the same basic principles that apply to aircraft should apply here. High speed and low speed flow will be different and changing the shape of one part alone, may have an undesired effect in other areas.
Want a new spoiler? Why not spoil the whole car:
TECHART Gallery>986>TECHART Widebody exterior