From my understanding the water that is sprayed inside of the headpipe through the three small screws is for tuning the exhaust, not necessarily cooling the headpipe itself. The water jacket around the headpipe is what actually cools the headpipe. Therefore if you severely limit the flow by blocking with a fcv except under throttle, the headpipe itself is not being cooled until the fcv opens. I could be completely wrong, I'm just explaining my logic.