How much power can the stock FA24 engine handle?

The engine is rarely the part that limits you, which is why this question has a misleading answer if you only talk about the block. The FA24 short block is generally considered safe to around 400whp on stock internals, with tuners advising real caution beyond 400 to 450. The connecting rods are the accepted weak link and they will bend under enough cylinder pressure, though they are meaningfully stronger than the FA20's rods in the older VA WRX. Past that range you are into forged internals and a built short block. But you will meet three other limits first, and in practice they matter far more. The stock turbo is comfortable into the low 300s at the wheels on ethanol and falls away past that. This is the first ceiling most owners actually reach, and going beyond it means a turbo, which drags injectors and a high pressure fuel pump behind it. The stock transmission is the one that catches people out. Tuners generally hold torque near 350 lb-ft to keep the factory gearbox healthy, and a Stage 2 ethanol car already sits at roughly 348 lb-ft. So at around 300whp you are not approaching that limit, you are sitting on it. The horsepower number still has headroom; the torque number does not. The stock fuel system runs out on ethanol before the engine does. The factory injectors and high pressure pump cap you well below the block's capability, which is why a 400whp build includes both. So the practical answer depends on what you are asking. Can the engine survive 400whp? Yes, generally, on good fuel with a competent tune. Can your car make 400whp without other major work? No - that requires a turbo, injectors, a high pressure pump, a front mount intercooler, ethanol and a clutch, and it puts the transmission somewhere it was not designed to be. The number that matters more than any of these is torque, not horsepower. A transmission does not experience horsepower; it experiences torque multiplied by gear ratio. That is why chasing a big low-rpm torque figure is the most effective way to shorten a VB drivetrain's life, and why a good tuner will build power up top rather than piling it on down low. For most owners the honest ceiling is around 300whp on ethanol. That sits inside the turbo's comfortable range, inside the transmission's torque limit, and nowhere near the engine's structural limit. It is the most power this car makes while every component is still doing what it was designed to do.