I don't think this is a violation of 406.4(D). The stainless steel cover complies. There is no possibility of debris or other objects being allowed into the box, due to a gap between the receptacle and the cover plate.
That section has nothing to do with preventing debris into the outlet box. Its purpose is to make sure that the attachment plug can be fully inserted into the receptacle. That cover will prevent full insertion with some types of attachment plugs and can lead to a poor connecting and damage if the load is high. Look at the new (2005) Exception #2 to this section. This permits a cover over the receptacle itself with a maximum thickness of 0.04 inches. This exeception was placed into the code over strong objections from UL. Their objections were based on the fact that the attachment plug cannot be fully inserted. The extra thickness of the device in this thread appears to far exceed 0.04".
Don
