WDLXTV_LIVE 0.5.1.1
With both rtorrent versions (the one built in the firmware, and the other version 0.9.2 which runs off my 500GB passport hard drive ext3 formatted) I get about twice your speed (1000-1200 kb/s which is my max download speed). I only download one file at a time (all other files are stopped no downloading or seeding), and limit my upload speed to <50kb/s (100 kb/s is my max upload speed). I use wireless connection and my router is about 10 metres away from the WD Live with no walls between them.
You should check if you have all the ports rtorrent uses open in your router.
Also you might have to tweak your rtorrent settings ("Download" and "Connection" tabs) maybe limit some options there to lessen the CPU usage.
WD Live working almost non-stop for about 3 whole years now, downloaded with rtorrent and played hundreds of movies, minimum power consumption, most useful device I've bought the last years.
Many thanks to B-rad and all the people who contributed all this time.