For me so far, suspension. The handling is so good from the factory, I didn't want to risk making it worse. The easy thing would have been to get really expensive dampers (JRZ, Ohlin, Moton, etc.) and call it a day, but I didn't want to spend $3k+ either.
Then Pobst wrote an article describing his experience track-testing a BRZ on RCE T2's, calling the handling balance magic.
http://www.sportscarmag-digital.com/...015?pg=22#pg22
Once I read that and saw they were well within my budget, I bought them. Big purchase, but no regrets. I did an open track day with them a few weeks ago and happy to report Pobst was on the money in his review (as usual).