Cat thieves are in and out often in a minute or less. I don't see an alert doing much good, by the time you can throw your shoes on and run out there they'll be gone, and there have been an alarming number of cases of thieves shooting at car owners who confront them.
IMHO the way to stop cat theft is by making it almost impossible to sell one for money unless you are a licensed dealer and have a paper trail. I would go so far as to make simple possession of more than a couple of them without paperwork a crime. Enforce severe penalties on any individual or business that buys cats without the required paperwork.
That is true, and an alarm by itself isn't going to do much, other than annoy the thief, but the person asking me to do this insures storage areas, where lots of vehicles are stored. They had to deal with one lot where 17 vehicles had their cats stolen in one night. In a situation like that, alarms would certainly be helpful in mitigating the amount of damage that can be done at one time. His idea is that his company will charge more to insure vehicles that have no deterrent installed. The alarm would be the least intrusive means for that. They would get a further discount if they introduce some system for making stealing the cat take more time and effort.
There is no solution that can totally prevent theft. Making it illegal to possess too many of something has never really served as a deterrent either. There are several ideas that have been floated, such as serializing the converters to link them to vehicles, but that does nothing for converters that are already on the market, unless the owner of the vehicle has an aftermarket serialization done. There are already systems, like "Leads online" where you can have companies that purchase things from the public list identifying information, along with serial numbers, on their database, where police can access it and sometimes catch the thief, and even less times recover the stolen property. That could go a long way, especially if the cat was stamped with the vin of the vehicle they were attached to. But, what happens to the aftermarket for vehicle parts? Could you no longer use a salvaged cat, since it will be serialized to a different vehicle?
Beyond that, where there is money to be made, you will find people that evade measures to catch them. You can't sell a stolen vehicle, yet people do it anyway. They have VIN numbers that are unique, but vehicles are stolen every day. Often they end up in other countries. If you can't stop people from shipping entire vehicles out of the country, which are very difficult to hide, how are you going to prevent catalytic converters from being shipped out disguised in a load of scrap steel, for instance?
Anyway, it's all well and good to talk about the best way to stop it, but those methods aren't in place yet. Until then, people have to do what they can to deter thieves themselves.