One of the more cost effective means of improving your handling would be upgrading your factory endlinks to a set of the Kartboy front and rear endlink combination!
Please note that these particular Kartboy endlinks are made to be used on the WRX sedan from 2002-2007. Save yourself a little money by purchasing these in a complete set rather than in individual sets.
Kartboy Front Endlinks p/n KB-017-F
Kartboy Rear Endlinks p/n KB-017-WRX-R