A centre box " is " a silencer . That's why cars have them fitted as standard , to quieten the exhaust along with the back box .
So what you're saying is . You've had a decat fitted , which is a straight through piece of pipe and another straight through piece of pipe attached to that :S , then that's attached to the original yozza exhaust ? A decat will always make it slightly louder . It did when i had mine fitted to my Yozza anyway .
If you could get some pics it would probably help to explain exactly what you've had done to your exhaust .