I'm pretty sure it's the same as previous years. 90+%. It's not anywhere near a bolt on solution. It's cheaper to trade the WRX in for an STi. Although there are other options, some stronger even than the STi's 6-speed and many cheaper. Most WRX owners, even ones who broke their trannys, probably shouldn't be considering 6-speed swaps.
That said, you should be aware that the tranny is not indestructible, even at stock power levels. Launching your car can cause serious damage if you mess up.
2015 STi: Still Stock
04 STi, 04 WRX: SOLD