I don't know what all Roush does with their cars, but this 07 has other stuff that's been branded, like the gauges.
Of course this is from a different model year and body style, maybe someone will be along and give you better tips than I can, but I'd think that there would be more evidence than the sidepipes.
One other thing that I think I notice from that pic is that the rear bumper doesn't have the Roush ground effects like I see in different Google pics, like this one for example.
If I had to guess, the seller (or a previous owner....) installed the side pipes on a run of the mill GT and called it a day.