If they want it bad enough, they are going to take it. If you have hood vents and I knew you had an alarm, all i would have to do is break them, reach my arm under them and find the hood sensor, then disable it and disconnect the battery.
If you had a secondary battery All i would have to do is to get into the car, fallow the wires and destroy the alarm box, another thing i could do is disconnect the horns and headlights, which would make it almost impossible for someone too notice.
If i wanted it REALLY bad, I would make a Ez-steel wiring harness, so once i gain hood access I can install the harness quickly and drive off, then deal with the alarm when i get to a shop or garage, (where i would discover the secondary battery)
you best bet is system that pages your cell phone whenever it detects something, or lojack.