C++ :: Intermediate Input / Output Buffer For Objects

The documentation of the class filebuf in the reference of cplusplus.com says:

Objects of this class may internally maintain an intermediate input buffer and/or an intermediate output buffer, where individual characters are read or written by i/o operations. These buffers are synchronized with the contents of the file once filled up, when explicitly requested to do so (sync), or when the object is closed.

Objects of this class may be explicitly made unbuffered by calling member pubsetbuf with both arguments set to zero (see member setbuf): Unbuffered file stream buffers perform the i/o operations directly on the file, without an intermediate buffer.

The C++ standard ensures that filebuf objects have an intermediate input/output buffer/s (i.e, the default constructor of the class filebuf creates the intermediate buffer/s)?

The standard C++ library only allows unbuffering filebuf objects (as the above quote says) but doesn't allow forcing filebuf objects to be buffered.

I have been seeying the concrete implementation code of the standard C++ library in my Windows Operating System (Windows 7 Ultimate 64 bits Service Pack 1) and it seems that fielbuf objects never uses intermediate input/output buffer/s, they use FILE streams of the standard C library instead to do the work. Are this FILE streams always buffered? If true, are they fully-buffered or line-buffered? what is the size of the buffers (perhaps macro BUFSIZ from <cstdio>)? and can I change this size?

I am worried about performance in reading and writing from/to files: if the default behaviour offers the best performance (perhaps if files are too large is better force buffering and choose a larger buffer size).