On my router I only have set-up a single-port forward that that is indeed port 21. The majority of routers do seem to have the ability to handle FTP traffic and protocols pretty well. If they don't the there are things to try without te need to mess about without the inside workings of the MyBook Live.
I have modified the vsftpd.conf file but only really to improve security. Through FTP I've hidden and denied access to the
Public directory, changed the banner to some quote, set max_per_ip to 2 and confihured vsftpd to delay showing
Invalid Login to unauthorised attempts for about a minute. Obviously keeping a copy of the original configuration file. I don't wish to change too much because firstly I did buy is as network addressed storage so have no desire to increase it's functionality by adding feature packs. Secondly, I think if I make too many changes then future updates from Western Digital may fail. I curious how many software modified MyBook Lives' may end up either not taking a future update or end up being bricked by a future official firmware update.
I use a dynamic DNS service DynDNS mostly because the router directly supports the use of this free service so I can find my home netowrk, but I guess you already know how to do this.
Now... Down to the issue of speed...
From a friend's house I transfered a short movie file from my phone to my MyBook Live and the statistics for that was 2390144 bytes, 78.24Kbyte/sec. I think the transfer speed is because of the slow speed of the phone.
What is your upload speed?
I think this is going to be a bit hard for me to answer because at my end it simply worked perfectly out-of-the-box. (Have you made alterations to how you MyBook Live's operating system behaves?)
What is your upload and download bandwidth of your Internet connection?
This next bit may sound stupid, but have you tried stopping the Twonky and iTunes servers and seeing if the transfer speed of the MBL's FTP server improves? Object of this excercise it to try the simple things first.
Really, me (and everyone else) need more information to try answer this.
(PS: What's the transfer speed right after the MBL's been rebooted?)