You can turbo the 03 and up, you just need to run a stand alone management instead of a piggyback. The 03's use a drive by wire that isnt compatible with a power FC.
you are right and wrong.
2003-2005 celica GTS's got the dbw. the GT celicas did NOT.
there isnt a standalone ecu option available for the 2003-2005 models.
you CAN turbo a 2003-2005 celica gts with DBW. you wont make that much power cause your tuning options are severly limited. Stafford fabrications has I believe 3 turbo 2003 celica GTS's. the power is NOT worth the $$ though.