-- embedded SFTP server, transfer speeds reported to be much slower than other protocols
-- using 5.8 <= SP 6   (or possibly 5.9 <= SP 4)

First, note that there are many factors that can affect performance.
-SFTP uses SSH encryption, a different animal than SSL used by FTPS; not a direct comparison
-some SFTP clients are better than others for testing. Filezilla has been reported as not suitable for performance testing since it showed high variability from run to run
-Any Java based SFTP server will perform differently than a native SFTP server
-other factors

That being said, we have a report that there is an increase in performance when one or both of these changes were made:
-new version of SFTP library that comes in 5.8 SP 7 and 5.9 SP 5
-upgrade from 5.8 to 5.9 SP 5 or later.  (may be other factors in 5.9 besdies SFTP library)

Recomendation, if you are bothered by slow performance in SFTP server, make sure you are on the latest service pack.  Also, consider an upgrade to 5.9 or later.