A few thoughts:
-I bet the effectiveness depends on where you have your shop. I seem to recall a story (LA Times many years back) of a jeweler in California who got robbed repeatedly because he "liked" shooting back and it was considered a challenge. Probably having lots of valuables had something to do with it, too. My memory may be faulty as to the details. Obviously most criminals would steer clear of you I'd think.
-It occurred to me that it might be considered impolite, like a sign saying "You will meet my lawyer if you try to defraud me" or "I will give you the cold shoulder if you act like a jerk". "I will kill you if you try to harm my child." True, but weird to just say it. Ie. you don't threaten people by default, you presume and expect politeness and appropriate behavior. Random people might have a bad reaction due to this subconscious "habit", without really disapproving per se. I do think most people understand the risks of a public business and the potential deterrent value.
I personally tend to keep a low profile about such things, but I haven't been in a similar situation. My 2 cents, anyhow...