when you say stock security system, do you mean the upgraded system that comes on the STi?
for the record i don't put much faith in alarms which make sound. no one pays attention to them anyway.
i prefer a fuel pump kill + starter kill that can't be defeated without hours of labor. they can always flat bed it away. but then the normal thug looking to joy ride or rip out the good bits won't be able to drive it off.