Essentially, the STI and WRX are the same... since the Torsen type diffs on the STI only kicks in when there is a torque difference (wheel begins to spin), and the WRX applies the brake on the offending wheel, effectively giving the same result - conservation of traction. However, if you drive like a maniac for an extended period of time, the brakes will heat up some. Considering BMW uses this exact same technique is heartening.
If you are going out for track days, then maybe you need to install an LSD.
If you car is a daily driver, I think the stock system is pretty good. Just do some suspension work to help the traction out.
There is this diff - A cusco for WRX 02-11 - for an R160
There are many diffs for R160's from Quaife, OBX, etc, but only the cusco lists the fitment for a 2011 WRX. So, while it is an R160, I would assume there is a fitment issue with newer R160's. If I find out a Quaife or better and OBX $ will go into my R160, then that will be my choice. Something will be available.