Yes it's true that if you apply full or partial carb heat, you neglect the full potential of engine output. A good example is on final approach - you descend at an aplied rate (say a 3 degree descent) with full carb heat set HOT, but before touch down (approx 100 feet) carb heat MUST be set to COLD. The reason for this is simples = if you are forced to do a go around, full engine output will be required to ascend, hence the reason for turning carb heat to cold - because carb heat set to HOT reduces output.
Simples :-p