IsElement() and nsIElement

So I'm looking at creating a non-virtual IsElement().

While examining the callsites, it seems like a lot of the use is in 
assertions in places where we want to be sure we're dealing with an 
Element node.  Would it make sense to create an nsIElement for now, 
sitting between nsIContent and nsGenericElement, to just enforce this 
sort of invariant in function signatures?  Or should we just make such 
functions take nsGenericElement arguments?  Or maybe that plus rename 
nsGenericElement to mozilla::dom::Element or something?

-Boris