Hi Andy - as well as paying professional tuners, it is possible to remap the new age imprezas (bug eye onwards) using free and open source software called Romraider (previously called Enginuity). It has been developed by fellow enthusiasts all over the world and is as powerful as the software professional tuners use such as Ecutek. Romraider also has an excellent support forum of users who provide advice and share experiences of remapping and tuning using this software.
I have used Romraider for 18 months now and with the necessary engine modifications have tuned my car, which is a daily driver, to over 400 bhp (using fuel additives and V-power).
If you'd like to know more about ECU remapping on the impreza there is also alot of information on the Romraider site.
www.romraider.com