There is another, cheaper option you might want to try. Whomever has the GT you're looking to buy may also have a V6 coupe of the same color without a spoiler on the lot. The dealership might be willing to exchange the entire trunklid between the two vehicles in order to make a sale. They might even do it later if they didn't have the correct color V6 coupe on the lot but got one in later.
The only caveat would be that after the trunklids are exchanged, the two vehicles would then have incorrect fake gas cap emblems. However, it might be possible to exchange those without too much trouble, as well.
I, too, dislike the rear spoiler and this is the first thing I'd ask the dealership about if I wanted to buy a GT off the lot.