In practice Qi chargers have just one operating power, either "within range" or "not within range", so whether you have a case or not won't affect charging rate

In practice Qi chargers have just one operating power, either "within range" or "not within range", so whether you have a case or not won't affect charging rate, but in theory RF power does scale with 1/distance². So you could argue that Qi standards engineers did have to limit our usable charging power because they knew cases would be used.