I'm late to this subject, you may have already fixed your issue. I replaced my spoiler for looks, got it off e-bay cheap, and painted. To my suprise, it drastically reduced buffeting. Even on the windiest days in MI, my Compass feels more in controll, then with the OEM spoiler.
I should also state that I have an OEM spoiler that I don't need. If anyone is interested in it, I would be happy to send it out to you for the pice of shipping. I'm not interested in charging a price for it, as less clutter is priceless.