I never understood what backward compatibility was met by windows api not supporting >260 chars file paths. It will work just in the same was if you pass any short path and no old application expects a long path anyway.
Your example isn't problematic API-wise, because CreateFileW doesn't need to care if you pass in 16 characters or 1600 - if it does, that is mostly a matter of refactoring and not inherent to how the function works. The real problem are APIs that inherently assume that you pass in a string of at most MAX_PATH characters, because you provide a pointer but no size, and the API is expected to write to that pointer. This affects most shell32 getter functions, e.g. SHGetKnownFolderPath.
But for functions outside of Windows itself, this is the exact reason why the long path feature is hidden behind an opt-in flag.
