I have been working on a 2012 Impreza WRX STI Sedan and over time I have added a Cusco rear Strut brace and SPT front Lower chassis brace. I also fitted a Group N transmission mount and factory STI short throw shifter and shifter bushing from Perrin as well as a Perrin M/cyl brace. We also have a front strut brace from SPT as well as a SPT Turbo heat shield.
I fitted a catback exhaust system from AVO however the owners wife did not like anything other than factory system so we went back to stock.
I have focused on things that will help to enhance the factory performance without risking warranty on the car. The chassis braces and Strut braces have been a good addition and it gave a noticeable reduction in flexing.
One other area I have spent a huge amount of time on is working to reduce Intake temps by fitting SPT heat shield which also has gold heat foil attached to the underside of shield. This led to significant change in temps around the intercooler. I also wrapped the lower portion of plastic intake that sits above the radiator which also helped to reduce heat transfer to intake. Finally I wrapped the intercooler, I would like to fit silicone intake hose in the future also.
Cusco rear Strut brace
Although this is not my car I have spent a lot of time working on it and I thought that the changes have been good so I thought I would post this thread