After pushing a TD04 as far as a daily driver would go...
Don't bother.
EJ255 will happily take a VF series turbo (get one off the same year STi with STi TMIC too) and it will provide all the power you need.
MY05 WRX (EJ205) with TD04 + STi TMIC + 816cc injectors + 2+bar of boost
MY05 WRX (EJ205) with VF34 + STi TMIC + 816cc injectors + 1.8 of boost
Less boost +40hp +40Nm.
Best bang for buck mods you can do. (EJ255 will just need an 06+ STi turbo)