Refering to http://www.samba.org/samba/docs/man/manpages-3/smb.conf.5.html
the better and correct syntax is
use sendfile = yes aio read size = 16384 aio write size = 16384
since sendfile can result in some problems with older clients I´am not quite sure about that The buffers and tcp options can be changed by using the socket option in the gui, the detault settings are taken quite conservative