If you're going to the trouble of installing a spoiler on an unspoiled trunk, might as well get something functional. As I understand it these cars have net aerodynamic lift - various claims hover around 100 pounds at the upper limit of legal highway speeds. The stock GT spoiler changes that by single digit amounts, if at all.
The Saleen S281 spoiler (or reproductions of it) have a pretty close to OEM look & supposedly come close to offsetting the lift entirely.
To head off any arguments this talk about lift comes from reading a lot of forum posts from a lot of years, my engineering education does not cover CFD and even if it did I don't have the models or equipment & none of this should be taken as more than an anecdote.
