Are thefts an issue in your area? Do you have personal experience with such theft?
https://www.cartalk.com/car-owning/how-much-does-it-cost-to-replace-catalytic-converter
My 2 cars are too low to easily allow cat theft… plus they are garaged. My truck has the cats tucked up pretty high so cutting them off is very difficult. It is an issue in my area.
My state has no emissions check nor is it one of the more “particular” states that require California emissions with the expensive catalytic convertors… So they tend to be replaced far more cheaply or not at all.
+1
In the weekly police blotter, they usually note 3 or 4 cat converter thefts over the preceding week. The stated estimates for replacement range from $300 to $1,800. These thefts seem to take place mostly in industrial areas, at night. So far, no cat thefts from cars that are garaged, but the cops are advising that people have their cats tack-welded to the pipe in order to make theft more difficult.
The police blotter also notes 2 or 3 vehicle thefts weekly, and I estimate that ~90% of them are stated as being vehicles that were left unlocked/keys or key fobs in the car/idling.
That’s not going to deter cat theft.
These thieves take a Saws-all and cut thru the pipe extensions on the cat, or cut the exhaust pipes before/after the cat.
Tester
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I agree.
Their other suggestion was to have an alarm sensor wired-into the cat to detect theft. 
I might ask the Lexus dealer about that suggestion in a few months when I am due for my first service.
2006 Honda accord 4 cyl. got converter stolen cost me around 800$ local muffler shop including o2 sensor.
The price depends on type and size of the converter.
Tester
Will the sensor and alarm cost about as much as a new cat? At some point the insurance against theft becomes too expensive, especially if your current car insurance covers it.