Well, considering that rear spoilers do nothing below speeds of about 80 and below, then it would just be something that looks cool, but not really functional. I'm not sure that there is any functional spoilers for these cars on the market, and unless you plan to spend a lot of time in triple digits, don't plan on a spoiler adding any "performance" or downforce to your ride persay. I would go with the 'stang spoiler, or even a factory bird spoiler. They seem to flow with the lines of the car the best.
Just my .02
These are highly engineered precision vehicles, the first step in diagnosing the problem is to strike the suspected offending part sharply and repeatedly with a blunt object, then re-test.