Magsafe Charging performance with magsafe compatible cases

I am just wondering. Magsafe charger is using induction charging technology. When we are talking about electric induction, of course the material that separate the coils is a factor but also very important is the distance between the coils. In the case of Magsafe charger for Iphone, the rating is for 15 W but, this rating is also valid when using original apple magsafe cases or is a rating when using the charger direct with phone ( no case between). If there is a degrease of power when using with a case, what will be the decrease? Did someone documented this type of test? It's not a big deal but I'm just curious if there is a influence in charging efficiency when using or not using Apple cases.