My impreza has an immense buildup of thick black soot just like your forester. It also burns a quart per 1700-2200 miles. Also, the rear of the car gets as black as a diesel after a few hundred miles.
There are many owners of 2011+ foresters reporting oil loss, as well as Imprezas.
There is a long thread for oil consumption in the impreza forum, I'd be delioghted if you chimed in with your account, as we are discussing oil consumption in fb and fa engines.