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These are still just rumors for the JDM models at this point. It's possible that the USDM WRX will keep the FA20 and the STI will get the FA24 since both engines are already certified in the US. It would be like Japan getting the EJ207 and the US getting the EJ257 since Americans love torque and "bigger is better".
Even then I would expect the next STI to be rated between 310-341 hp. I don't think Subaru will immediately outdo the S209's power figure.. they'll probably wait for a mid-cycle refresh.
The advantage to the FA engine is that it has a broader torque curve (thanks to the EL header, twin scroll turbo, higher compression, DI etc.) and responds better to bolt-ons & ethanol. The WRX's FA20 already makes 320+ whp with a "stage 1 flex fuel tune" with everything else being mechanically stock (just an AccessPort, flex fuel kit & tune). I'm guessing an STI version will be capable of at least 350 whp with a similar setup.
Not to mention the improved fuel economy as an added bonus. I got better fuel economy in my '16 WRX running E50 than I do in my stock '19 STI
Subaru has the potential to make a great car.. only time will tell if they actually do..
I wish the S209 would have received the EJ207. Instead we got the same EJ257 (without forged pistons), UEL header, and slightly larger HKS single scroll, journal bearing turbo.