The heat crossovers are for cold start and driveability. They heat up the intake charge to keep the fuel vaporized and keep it from condensing on its way to the head and cylinder.
If you want more power for the track, block them off. Heat makes things EXPAND. By keeping the air/fuel mixture cooler (once the engine is warmed up) allows more air and fuel into the cylinder by packing more air and fuel while it is cooler and more 'condensed' . More air and fuel makes more power. If you heat it up, it expands and takes up more room for the same amount of air/fuel than when it is cool. That is why the cars seem to run better when the outside temp drops between 40°F - 60°F. They are packing more air and fuel into the cylinder than when it is hotter and making more power.
