I agree with john, and will share my personal experience.
I had the same problems with my z, exhaust fumes when I would cruise at high speeds with the windows open, and faint fumes with windows closed. I fixed it by going into the hatch area and sealing up all the little holes I could find, and by replacing the tail light gaskets.
That was it! for me at least.
My advice to you; Fix the leaks first, and then if you really want a roof scoop, install it. But don't install a roof scoop and expect it to make your fumes disapear.