It depends on which stream you want to test, ie. upload or download. It also depends if you have write (upload) privileges on the FTP account, as that would give a more accurate speed test as opposed to P2P.
What I recommend doing is downloading this torrent:
it has many good peers, I was able to get 11.2MB/s download. That will obviously test the server's download speed, now to test the upload speed we will use my cunning method for this, and that is Rapidshare. Make a collector's account for yourself (it's free), after logging in go to the remote uploads tab and input this url:
This will give you a very good idea of what your seedbox's upload and download speed is.
LOGIN - your ftp username
PASSWORD - your ftp password
DOMAIN - the ftp server you connect to (hostname or IP address)
PATH_TO_FILE - this is the path to file you see when connecting through your ftp client, for example /downloads/file9384.rar