If Subaru wanted to approach the current bracket of V8's it would need to go with a bigger engine, not a smaller one. 3.6 liter boxer would be a good start, but instead we're seeing a 2.0 boxer which I seriously doubt will be pushing 400hp anytime soon.
Be happy to eat my words, but after that new Mercedes debuted with 355hp on a 2.0 direct injected engine which took 23lbs of boost and a 55k pricetag...I'm seriously doubting Subaru will be able to race much of anything in the years ahead outside of an angsty Camry or some mom bringing it in her Previa minivan. Bring your a-game though because even those are decent powered at this point.
No replacement for displacement, at a certain point. So Subaru's are fun and a thrill to drive, but they're getting trounced with modern innovation and cubic inches.