seeding make WoW lag.
Get a better BT Client where you can limit the upload of your seed. Put it to something like 1/4 to 1/2 of your total upload speed and leave it there seeding PS forever with minimun lag/distruption to anything else
For example got my server on all day and started playing the game yesterday. Loving it so I've allocated 20k of upload of my bandwith to seed it forever

I recommend utorrent or bittornado (Azurreus is quite good also but I'm not a fan of a heavy graphic program like that)