Stock Subaru wheel offset is between +45mm to +53mm. This means that the +15mm offset will push the wheel outward about 30mm (1.25") from the center of the vehicle. On some vehicles this will cause interference with the fender depending on what size of tire is being run.
What is wheel offset?.
The offset of a wheel is the distance from its hub mounting surface to the center line of the wheel.
Zero Wheel Offset.
The hub mounting surface is in line with the center line of the wheel
Positive Wheel Offset.
The hub mounting surface is in front (more toward the street side) of the center line of the wheel.