Actually when writing to an 'AF:' path, we initially resolve a PI Point Data Reference to the underlying tag, and cache that value. The PI SDK is then used for the write, so buffering and fanning should work equivalently for both paths.
In terms of the underlying data access mechanisms: for the PITimeSeries methods, the PI SDK and AF SDK are used exclusively. For the PISoap services, the AF SDK is used for Event Frame search/retrieval and PI OLEDB Enterprise is used for AF Element/Template/Attribute search and retrieval. The reason for the difference is that the OLEDBE Event Frame capabilities were not yet complete when we began developing the Event Frame capabilities in PI Web Services.
Hope this helps!
PI Web Services Team