Sounds like the file you're trying to download simply isn't seeded to 100%. ARE there any seeders in that torrent's peer list with 100% ? If no one with 100% is around to give it to you, you obviously can't get it. Or maybe the number of people with the total 100% is fewer, and they have slower connections than the rest of the people in the swarm. If you're sure it's seeded 100%, you'll just have to be patient and wait for it to complete.
With that kind of connection speeds, there isn't much you need to do with a torrent client in terms of configuration, short of adjusting your queuing options to suit your personal preferences, and making sure your torrenting ports are properly forwarded.
Quick guide to Tixati optimizing and port forwarding:
http://www.tixati.com/optimize/