When I click on your link, it takes me to a wind deflector for a Mustang Convertible. So, I'll give my $.02 from what I experienced.
I made a roof deflector for my '04 Jeep Grand Cherokee out of Lexan and aluminum square tubing. I was pulling a '08 Rockwood Roo 23SS, about 5600# worth. Normally, I got about 12 or so mpg on a trip. Using the deflector to make a trip from Western MD to Myrtle Beach, I was pushing close to 14 mpg.
I've read all the other posts here and on other forums on the pros and cons of deflectors. Personally, I say try it and see if it works for you. On a short trip, 2 hrs or so, I don't think it would be worth it for me. However, on a long trip, it probably saved me close to a 1/2 tank of gas. And at gas prices, it was worth it in my book.
Let the nay-sayers chime in!!!!