That makes absolutely NO sense whatsoever. When I am in the mood to rob (as by my stats isn't that often, but still)... I go into a hood regardless. So whether or not you are level 105 or level 145, whether you have 125 mafia or 5000 mafia, I'm coming in.
And you only do away with the "500 mafia" attacks. If you are over level 110, 112, etc you are still pray to the other goals.
And - you ARE weaker with one less mafia.