Running the cowl or not running the cowl will not make a noticeable difference. RC carbs run pretty cool due to evaporating fuel, in fact in certain conditions the intake tube can get frost on it and in other conditions almost always has moisture on it from the cooling effect. This is much ado about nothing. When you run the mixture a couple of hundred rpm rich to compensate for the fuel level changing you've already blown away far more than running an intake duct would gain you.