honestly I don't think it would be that hard to install. From what i've seen, the non spoilered GPs still had the cutout in the lid frame for the spoiler holes. Look at the decklid closely when you pull it off, you'll see what I MEAN. IIRC, the holes were drilled in the center of the holes in the frame. You may need to make a couple of the holes bigger but it should be straight forward.

Make sure to get the studs/nuts for the spoiler too, it'll make installing it easier too.