I'm not sure why you're thinking PHP's autoloading is a performance issue? Especially regarding composer's ability to dump a optimized autoloading map it shouldn't be a problem.

I agree, this neat feature should definitely be documented as it's rather a hack/trick.
The other thing you've mentioned is actual a rather big code change I guess.
It's basically a good idea and the way to go if we want to have IDE's auto completion.