it's not a question of "if you can or cannot use the stock WRX top mount i/c" it's why would you?
it's a matter of efficiency. The STI i/c may not flow much MORE air compared to the WRX, BUT it's larger so it does flows more and has a larger surface area which allows for more efficient cooling which is something you should be concerned about. when your pushing almost as much air as a small 16g you would definitely benefit from upgrading your intercooler, injectors and fuel pump it's just that simple. if you didn't do these upgrades your tuner would have a hard time giving you a safe tune and he would be limited to boost, fuel and timing which would restrict your final horsepower number and not having enough fuel during a boost spike at high RPM could cause damage to your engine. why risk it?