How does the venerable buff book know this? Because the turbo'ed flat-four has already appeared in the Japanese domestic market Legacy, making 296 horsepower and 295 pound-feet of torque, mated to a continuously variable transmission. C/D says that engine differs from the one in the BRZ in that it uses Subaru's own direct injection system rather than one coming from Toyota.
In the standard WRX, horsepower numbers are expected to be a bit lower than in the JDM Legacy, though the STI version will probably see power boosted. C/D says the new 2.0-liter engine is supplanting the 2.5-liter Subaru has been using due to fuel economy considerations, though there's also something to the notion of consolidating powertrains to reap greater efficiencies in manufacturing.