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Engine block heaters weren't designed to operate in free air, but immersed in coolant inside the engine block.
That being said, a properly installed block heater would still fire off your cat sooner. After all, waste heat energy is being shoved out the exhaust, that would have otherwise gone to heat up the engine block. How much sooner?... Eh. Tempted to say that you'd shave a few seconds off the cat's warm-up time.
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