I always thought that car alarm manufacturers had it back to front.
A few years ago, I made my own alarm (I was studying electronics and bored at work!) and came to the conclusion that the sirens are in the wrong place.
So, I mounted 2 x 110dB sirens inside the car, under the dash.
So loud inside that you couldn't actually think.
An alarm though, is only as good as it's installation. Do it properly and the world is a sunny place. Do it badly and it's a bad time waiting to happen!
All cars from a certain year (96 or 99, can't remember now) should have an Ecu based immobiliser on it. Unless it's not uk spec.
So it should stop the good old twoc' brigade but nicking your latest one direction cassette? Nowt helpful there. So if it'd make you sleep better, go for it.
Not so sure on accessories and can bus though....