The bang on shutdown is a result of you turning off the ignition......which causes the spark in the cylinder to stop - while the engine continues to turn and pull air through the carb....where it picks up fuel.....then the fuel/air mix gets pumped through the engine and into the exhaust.  The unburned fuel/air mix is now in the hot exhaust system....and within a few seconds after shutdown the fuel/air mix has become hot enough to ignite and create the explosion in the exhaust system.  If the fuel/air mix is a richer....the explosion is less violent and more of a rumble - if the mixture is a bit leaner the explosion is louder and more violent.

I believe you should be adjusting your carb for how it runs when the key is turned on and the engine is running - not for what it does when you turn the key off.  If your idle is high - it pumps more fuel/air into the exhaust when you turn the key off....so the  noise will be louder.  If you shut the bike off immediately after getting off the highway - the exhaust system will still be hot and more apt to ignite the mixture (allowing the engine to idle for half a minute might let the exhaust cool down enough to avoid the bang).