The splitter is an add on but it won't fit the GT front, you have to change the front valence to the CS one. The CS is flat on the bottom and hides the painted part below the lower opening.
The conversion part from Ford comes with the fog lights in the outer openings, on the Laguna Seca model they're air ducts for the front brakes.
This one is the race version of the splitter. I'm hearing that it isn't street legal because if you hit someone with it, it could do some serious damage at the ankles.