My top tip would be to get a remap done - this will probably require a replacement ECU as the ECU's from the factory are locked, but there is a lot of extra performance to be wrung from the standard engine.
There is also a supercharger conversion kit being planned for the FT86 which should also work on the BRZ but the car is still quite new and they are sorting out the bugs and issues.
There's a place here in Sydney that specializes in Subaru's, they have a BRZ race monster, will get their name for you, they'll be able to tell you what's best.