My 2 cents...
If you're wanting 300hp (225kw) atw.. You will need to do a lot more than chuck an STI ECU, boost increase and some yellow injectors at it..
You are talking
bigger turbo
Exhaust
injectors
intercooler
ECU
uprated fuel pump
FPR
possibly forged pistons
Dyno tune
I would say your standard clutch would not like that sort of power so upgrade that too.
If your wanting power around the 300hp ATW mark then you really are heading into spending larger amounts of cash on an older WRX.
Im not saying dont do it but what i am saying is to think very carefully about your power goals. costs can spiral out of control very quickly.