I've seen a picture on TGR of that same generation outback with a skybox 18 on it (same length as the 21, I believe) and the poster said it does clear his hatch. However, there were 2 caveats.
1. He doesn't have the spoiler on his hatch, i don't know if you do.
2. He was using thule crossbars instead of the factory crossbars. This gets the box up a little higher and gives you more clearance.
So, to answer your question, maybe. If you have the spoiler, I doubt it'll fit without hitting. If you're using the factor crossbars, I'm not sure if it'll clear or not.