higher octane than your car is tuned for just makes you run rich. 93 in a car tuned for 87 is not going to hurt anything (running rich is pretty safe in moderation) besides maybe make your catalytic converter die a little sooner. It's not going to add anything though. The WRX is tuned for 91 octane from the factory. The factory map runs rich already, there's gains to be made with a re-tune even sticking with 91.
The benefits of high octane gas demand tuning to exploit on our cars. Some people like to have multiple maps to run based on what gas they have but this can be dangerous since you will tend to get a mix of gas when you do it. And gas doesn't stop at 93. There are several stations that carry 94 mattering on where you live. Some places, even 95. "Race gas" is typically available near race tracks at 100 to 104 octane.
2015 STi: Still Stock
04 STi, 04 WRX: SOLD