As for the signs... I've been in the security alarm industry before. Many houses are not adequately protected by the "free promotion" systems pushed on them by salesmen. Security systems CAN be a very powerful tool, but it can also be just a mild annoyance to anyone with knowledge of how they work unless the homeowner is REALLY familiar with how to make that system work best for them. With that being said, some of those systems have as much as 30 second entry delays. That gives anyone determined to get in 30 seconds to find the main control panel (obviously not at my house, but there is still an entry delay and someone with the know-how could keep the siren blaring down to a minimum of 10 or 15 seconds). If he can find it, then it's game over for security system. Not to mention signs invite them to cut the phone line before they break in, etc. Now take same person, they have no knowledge that you have security system. They kick the door in, keypad goes off, then 5 seconds later siren goes off. They weren't counting on that, and they get an "Oh, ****" moment, hopefully causing them to run off. Like I said, signs may be good to deter the amatuers... but it tells the pros to be prepared. That's a long way from leaving the door open and inviting them in.
Again , if these guys are so determined and dedicated .... This will work for you ONCE.
Once they have attempted to break in , get spooked by the surprise and run off ... they will be back and now the element of surprise is gone. You are right back to the point of " they know I have an alarm now, what it sounds like , how it reacts etc....
Unless you have some security camera's that can tape them doing it , then to the police it's just "attempted burglary" in which case it never happened and they can't catch the guys who did it ... leaving them out there for a second chance.
Also keep in mind your response to the freebee alarm systems ... If they make plans to account for the basic setups or false front stickers ... then they are going to be surprised anyway ... At the same time it will keep OTHER ameuters at bay.