I don't keep anything in my cars. They did, however, steal the head unit to my AVC/R... which is useless without the solenoids and sensors.
I have no knowledge of if it's even possible to incorporate impact sensors on the factory alarm, nor why you'd even bother. I bought a high-end 2-way alarm shortly after my car was broken into.
I love elitists who aren't really elite in any way... they amuse me greatly.