I do not plan to add any additional spoilers to mine.  From hearing the engineers talk about it, the little lip spoiler on the rear was designed to provide a balanced amount of down force at higher speeds and is totally functional.  I don't run mine much over 100mph but changing the rear spoiler is more eye candy than function and could result in instability on a well-tuned chassis.  If you're a street racer, do what you like because it won't matter much.  It could add drag at highway speeds and affect fuel mileage if that matters.