I have one on my recently-retired Civic Wagon. While I didn't do any accurate testing, it did seem to help on the highway. My theory was that it was directing air around the smoother sides of the car instead allowing it to cause drag on the rough undercarriage. I was careful not to make it too big, that would increase downforce and drag. Mine stuck out just slightly more than the bumper. Make a cardboard prototype and see if it works on your Camry! A vertical air dam is another approach.
