I've never heard that one before, I'm running a pair of open renegade cans on mine and I'm not getting any backfiring on the overrun, I was however, getting back firing, before changing my faulty ckps/stator assembly.
I think the mechanic is talking about backpressure but most bikers are running non legal exhausts and I can't believe they are suffering from backfiring on overrun.
Can you try putting the stock cans back on and see if that clears it up, it helped mine before I found my fault, if it doesn't sort out your backfiring, then I'd say you have a sensor fault.