I had a Yakima roof rack that I used with our Forester. It was the longest one they made, but was only 1/2 wide. Worked perfectly on the Forester, but was too long for the Tribeca (the back door wouldn't open up-the spoiler would rub). So, I picked up this carrier. It was sold by Subaru for the Forester. Original price was $280 or so, I found it on Craigslist locally for $80. I had to drill new holes into it to get it to fit the fixed Tribeca cross-bars, but it worked perfectly on our recent trip from NY to SC.
We made the trip this year and had the carrier on top, and a 6-speed transmission package in the back (400lbs) plus our luggage. New oil change and new Perrin air filter. We averaged 20.4 MPG over the 1850 mile trip. That includes a week of travel in Myrtle Beach and at least 3 hours of stop and go traffic on 95.
We made the same trip last year without the carrier, and without the transmission (-400lbs) and averaged 23.1 mpg. SO, there is a difference, but not that big of a difference.