Well, the three fastest WRX's in the world are all AT cars, but they are not your every day transmission obviously. It's easier to go from MT to AT than the other way around. However, I believe the transmissions are different lengths, so you will need a new driveshaft if that is the case. But, 283 whp is not that much. I have more than that and still have the stock transmission and clutch with 50k miles on them. As long as you drive like you have some sense, you will be fine with what you have.
But, you can get the STi RA gearset for $1300, which is at least what you would spend for swapping in a AT. I would just do that, unless you just don't like a MT.