https://blog.torproject.org/blog/bittorrent-over-tor-isnt-go...
If you're using a Linux box, you can use iptables to force all TCP traffic through Tor, dropping everything else. Then make sure the box doesn't know it's "public" IP address, only it's NAT'ed one. Even if it does a call out to something like http://whatismyip.com/ in order to determine it's external IP address, it wont get the real one because that traffic will have been forced out through Tor.
See: https://trac.torproject.org/projects/tor/wiki/doc/Transparen...
Speed wise, Tor appears to be slow, but that is just latency. For throughput it's fine. Especially if you're connecting to lots of different hosts over lots of different Tor circuits, as happens with Bittorrent.
But yes, the Tor Project doesn't want you to be using Bittorrent over Tor.