What are the advantages and disadvantages of leaving the open hole or installing the JCW plug for the airbox?

 

I removed the plug when I installed mine, I figured any extra available airflow is a good thing.

As an experiment I blocked the opening with a snug fitting stopper from inside the airbox to see what the difference would be. I noticed the SC whine was a bit more attenuated before the flap opened at 4500rpm, making the opening transition sound more dramatic. I could not notice any performance differences. I removed the stopper afterwards.