If you can get the straps to the hull raisers tight enough you won't need the bow/stern tie downs for short trips even on the highway. But if you are taking a longer trip I would use them for safety.

The hull raiser aero do not work well with the round bar you need to get the regular hullraiser for round bar. I have the Yakima round bar and had some hullraiser aeros and they do not hold well at all. They will slide back and fourth and the rubber grip underneath will come out allowing further movement during travel . I learned this first hand. I have since purchased the regular hullraiser mount to attach still not a super tight clamp down . The THULE bars and kayak mounts are much better. I have them on my second vehicle and no slippage issue and they hold tight.

The Hullraiser Aero will fit MOST factory bars. Certain factory crossbars like those found on the Nissan XTerra or the Toyota FJ Cruiser may not fit because they are ridiculously oversized. Check Yakima's website (www.yakima.com) to be absolutely certain for your vehicle. But in my experience (I sell car racks), I've never seen a fit issue with Subaru factory crossbars as long as you have the Aero version.

Yakima actually makes two different Hullraisers...the Hullraiser and the Hullraiser Aero. The Hullraiser Aero is made to work on any type of crossbar be it round, square, elliptical shaped, etc. due to it's Universal Mounting hardware. The Outbacks bars are elliptical but you should have no problems fitting the Hullraiser Aero on your Subaru. They will most definitely fit and work very well. However, the basic Hullraiser won't fit (will only fit on round or square bars). You will need the Hullraiser Aero.