This is a really interesting discussion, and I quite like the distinction you make between "most efficient" and "most similar" representation in the target language. From that perspective, it makes sense to me that CoffeeScript is very much a transpiler, while Closure is very much a compiler, which meshes with common terminology.

But it seems to me that these should be considered different kinds of optimization, rather than fundamentally different things, eg. `xpiler -Oreadability` versus `xpiler -Oefficiency`.