Cosworth bearings and APS sr56 Turbo Aftermarket,
would the stock fuel pump and fuel injectors be efficient
for the 500 to 550 hp increase or should l exchange the
fuel injectors and pump with a higher flow?

No l think you would need to go pretty big(800cc+) for that turbo and definitely a good fuel pump. Most Subaru guy is use Walbro pumps. Stock injectors are only good for up to a 16G maybe a 18G if tuned concervitively. Buthe way you will have to tune very aggressively and use race fuel to reach your power goals with that turbo. l would look to a rotated 35R kit to get there.

l dont think the stock fuel pump and injectors would be enough.

also, l dont think the stock axles and gear box would be enough. at that kind of power level, l believe it is possible to break an axle from a hard launch.

it will be very $$$$ to play in that power range with a STi. perhaps you should consider a large displacement V8 car like a corvette instead?

you need a better pump and bigger injecters all stock injecters and pumps are usually rated for stock stuff so go bigger. l wouldnt worry about anything else at the moment, l have a friend with a 800hp ST l and he has not had to replace anything but a joint (only cause he dropped the clutch at 5k rpms) ST l is are very very tuff cars.