My 944 exhaust (with catalytic converter) gives off a stinky sulphurous odor when you stand behind it, as many cars do, but you do not smell it inside of the car normally while going down the road. It does not smoke, but it does have an onerous smell.
However, when I have the rear side of the sunroof tilted open and I am decelerating in any gear, that noxious smell is drawn into the car through the sunroof. I think that the low pressure area created around the rear of the car and the wing, along with the deceleration, helps to draw the exhaust odor into the open sunroof.
By extension, it seems like exhaust odor could also be drawn in through the side or rear windows of the GT under these conditions as well. Does your problem exist with windows open or closed?