THe tranny's go if you rod the s&*# out of them or have way to much horsepower. But you can go get a Ra gear set and that fixes that problem. I mean you have to worry about crank walk and we have to worry about tranny's. Every car has a certain problem. If you dont mod it too much i bet the tranny would last.
I dont really know that much but i would say that 350 is a bit iffy from what i have heard. If your doing 5k launches it would toast your clutch and transmission, If you drive it normal it will last a while. I will say it wont break as much as a dsm. You dont have as many maintenance issues. It will also be tough to get to 350 horse without a good sum of money. My dsm broke all the time and my wrx is working pretty good right now.
The tranny issue is overblown like the crank walking issue. I wouldn't worry about it. If you lasted through a DSM you will last through an Impreza. It is the 7th most reliable car after all according to Consumer Reports.