If I'm not mistaken cable connections do share upstream and downstream so one does affect the other, but the most it should take away is 256K.

The best is advice is to go get the thing from someplace with a 30-day return policy and check it out. No one will be able to tell you what to expect but you.

Also, when your friedns download from you, are they using the same PC that you're seeing slowdowns on, or do you see a performance hit housewide? What kind of router do you have? I sometimes wonder if old routers can handle all that continuous traffic and still serve all the nodes effectively?