.pyc files have tags to specify details about them (e.g. were they compiled with -OO), so this isn't an "all or nothing" option, nor does it require a different file extension. There just needs to be an appropriate finder that knows how to recognize a .pyc file with the appropriate tag that can be used, and then a loader that knows how to read that .pyc.
Nope, we actually cache directory contents so file lookup existence is essentially free (this is why importlib.invalidate_caches() exists specifically to work around when the timestamp is too coarse for a directory content mutation).
-Brett