The Ej255 can handle 300whp and the Ej257 can handle 400whp both with stock internals with an upgraded fuel management and a tune. while the Ej257 has a black nitride crank and forged rods the ej255 only has cast steel rods and crank leading to rod bearing failure causing a decreased of hp you can make. from factory the Ej255 makes 260hp where the Ej257 makes 310hp. When you want to start making power both the engine cases are the same so for starters upgrade both motors with H beam forged rods and get the black nitride crank in the Ej255. Next would be to upgrade your pistons to either forged or hypereutectic.
Once you’ve got the new parts installed with some ARP rod bolts your motor could handle up to 500whp with supporting turbo and fuel modifications. The next weakest link would be the engine case this is when you would get your motor machined with closed decks. This is where reliability starts to trend down. Some ways to decrease the chance of a failure is to add a coolant reroute for cylinder 4, from factory cylinder 4 gets the short end of the stick causing it to heat up a lot faster than the other 3 cylinders. The biggest failure with turbocharged motors from Subaru is with modification they run lean, leading to predetonation so run as much fuel as you need to run a little rich and keep up with the full supply your motor needs to scale properly with your turbo. At this point your motor is gonna start getting close to making 650whp.
When you reach over 400whp Your gunna run into some other non motor issues like the drive train will start to break down fail. Commonly you’ll find high horsepower Subaru’s running custom built transmissions, driveshafts, front/rear axels, and rear differential. Definitely make sure you have the stopping power to bring the car to a halt so running Brembo’s or Wilwood break calipers and rotors are a must. Finally you’ll want the suspension to hold up to the weight transfer of the car and handle at its best under acceleration.