from what ive heard its starting base model 24k i dont think it will increase 20k just for sti if that is the case i will buy the base model and tune it myself. I think it will be like 30k max. but i thnk we will get about 80hp too. It will be sick!!!It better be turbocharged. A BRZ STi will end up costing $7500+ on top of the regular BRZ base price. If Subaru wants any luck in selling it, for say $38-44k (guesstimate), a significant power increase is no question. 80-100HP is nice, but at max I think we'll get 80.
Along with power increase we might see a CF roof, bigger brakes, lighter wheels, better tires, upgraded suspension, aero kit & some others.