I think you should be just fine. I have a Chesapeake 17 and carry it on a Chevy Impala, the widest I can get my bars is 30" and it holds fine jst make sure you use bow and stern lines. Subaru is a great option it will hang over a little but it should be no problem.

Jason

I had a 21' tandem on my Volvo XC with no problem and think I have a similar spread on the racks.  Bow Stern lines are VERY good advice.  Also, you want to have a flag on the stern.  I have a reflective one that my wife sewed for me that lives on the boat.

No worries with your length and an Outback. As others have said just be sure to have bow and stern lines and remember to snug them but not too tight. With that length boat and the lack of support, past the bar spread, you don't want to stress/flex the boat too much. I have put a long and heavy canoe on top of my Corolla with no problem, driving on bad Montana roads or 70 mph. I have had no problems what so ever.

As others have said, just make sure you use bow and stern lines.  I've seen 18' yaks on top of VW bugs at 70 mph. 

I carry my 18' 7" northbay on an accord with about the same amount of spread as you can get on the outback.  Never had any issues.

FrankP

PS:  Bow and Stern lines aren't good advice, they're a necessity.  A friend just had his 13' boat rip the rack off his roof and destroy the back half of his car to the tune of $3500 in repair damage.  Caved his roof in too...

Barely scratched his boat but jacked his car up pretty badly.  Bow and Stern lines are a necessity.  He was only going 45 mph at the time and just caught a bit of wind.

FrankP