Loud cans do help, especially in heavy traffic when filtering.
Only when the cage drivers have their window down and haven't got their radio blaring. It's true that modern cars are so insulated you can't hear anything outside until its too late. Modern day trucks are also getting the same inside - too quiet and you can barely hear bikes coming until they're past you.
I personally hate the standard quiet cans - they sound like hoovers and you can't hear bikes coming. I only use one for the MOT. Then it gets taken off afterwards.
If you are driving a cage, have the air con going, radio and windows up full, then you
will never hear a bike behind you sporting a standard hoover can.
Yet boy racers are allowed to use any exhaust system they want on their cars no matter how loud it is.
Not very fair....