The speed also depends on the connection speed of the seeders allowing you to download from them. This will vary during the day as different ones go on and off line. I generally download overnight, relieves lots of aggravation.
Yes that's the absolute best you will get its usually a lot under that...the speed from other peers and the number of seeds you have will affect the speed as well so the downloads in utorrent will always be a bit slower try turning down the upload speed but not completely to 0 though.
Also, your said internet speed is never going to actually be 256kbps. That 256kbps I assume is the top speed achievable (probably only with ridiculous computer specs)