Among the hype, there's a lot of speculation swirling around this car... especially concerning a power increase. Something fans have been asking for awhile. This won't come in the form of forced induction like many assume. The BRZ STI (officially dubbed, regardless of it's specs and features, will still be naturally aspirated.
When Subaru Tecnica International releases a complete car, it's to bring out and improve the level of how the car drives. Japan has STI versions of the WRX, Legacy, Forester, BRZ, and countless other models since they started with the iconic Impreza 22B in 1998. You could argue that Japan already has a "BRZ STI" with the same tS name, which shares a nearly identical rear wing, aero kit, and upgraded suspension/brakes. These are features we'll likely see on the new BRZ tS because this is what Subaru feels the focus of the BRZ has been: Handling Response.
Each time the BRZ has received an upgrade, Subaru has made it a point to emphasize the driving characteristics of this car. By sacrificing their coveted All-Wheel Drive system, Subaru managed to place their Boxer Engine lower and further back in the chassis to further balance this lightweight Rear-Wheel Drive sports coupe.
As a drivers car, the BRZ has always been praised for the things it excels in. It's handling, balance, and fun-to-drive nature have been at the core of it's design philosophy. People who buy this car understand that and want that. The "STI guy" has been on a power trip for over a decade. That, in tow, has given the STI badge in the United States an idea of what is expected: power. But that's not the point of the BRZ, so it logically won't be the point of the BRZ tS.
Even the Japan-released BRZ tS that I mentioned before doesn't have much more in the way of increased power under the hood. Naturally Aspirated power has been part of the design philosophy on the BRZ since it's conception, so if any more power is to come from the factory, it'll still be in that form. With Subaru's latest revamp of the BRZ engine has new valves, camshaft, and cylinder heads, as well as a new aluminum intake manifold, and redesigned exhaust manifold. These improvements only managed to wring an extra 5 horsepower from the BRZ, but many speculated that after making those many changes that more power should've been available. A STI-tuned version of the FA20 engine would likely bring out more of those capabilities under the hood. In a car that weighs under 2,800 lbs, adding even 10 or 15 horsepower would be noticeable and the improvements made to it make that kind of territory very accessible, but that won't be the case with this 2018 release.