Emphatically, YES, direct I/O is of considerable benefit to LGWR. Without it, LGWR has to do partial block writes as explained in one of the Ixora web tips under the heading Partial Block Writes.

However, the statement in the manual page is not correct. It would be better to say that direct I/O benefits all I/O for which file system buffering is not required. The only Oracle files that benefit from file system buffering are the control files. So direct I/O should be used for all other Oracle files.

I am on Solaris 2.6, and I get the following information from the manual about mount_ufs:

    forcedirectio is a performance option that benefits only from large sequencial data transfers.