Blocking the heat crossover, IN the head.

In street applications, you want the best atomization possible for good drivability and mileage. In a race application, such as aftermarket heads and intakes with no passage, fuel atomization isn't as important.

I'm not saying it "might not" pick up 1 or 2 HP under the right circumstances. But it's very unlikely in a street car. I know for me there are a "TON" of other places I would look other than the cross over. It's just not worth the extra effort.