I still say stick with the 335i. Your car is spooling up at about 1800 RPM's and a well tuned, well modded WRX will maybe start spooling at about 2500 RPM. If you are able to work on your own car then it might be worth your time to get a WRX, but like most BMW owners I know they have no mechanic abilities.
I don't know how much you owe on that 335i but do the math and think twice. I love my Subaru and you might love Subaru as well but it is like going backwards. If the math works out in the WRX favor do it but mod wisely and don't think it is easy. Better just hire a real pro and get it tuned right. Modding a WRX is not that hard if you know what you are doing, but you need to have tools and some experience turning a wrench.
This is just my humble opinion and don't want to scare you away. Just think about it long and hard which I am sure you will.