echo Stants statement - it appears to be a personal thing - looking at a 330 type option for yourself could be good - hi flow panel filter eg Cosworth just as long as its oil free that is fine, fuel pump RCM's are getting better feedback currently than Walbro, intake pipe samco, zero sports prodrive etc, rear silencer and remap
here is an example of prodrive option
http://www.subarupartsstore.co.uk/product/Impreza_WRX_STI_2.5_2008_-_onwards_Performance_Pack_SACC2811