well, i've got a 5mb/s connection, and i regularly hit 600-603kB/s on my torrents, so it's possible to get close to 100% of your connection for sure
make sure your half-open connections aren't limited by windows, and that you have your ports triggered or forwarded correctly, and are using as many connections as possible without crashing your router.
for example, in the picture above, i was downloading 570kB/s, but downloading from 175 seperate sources at once. if your connections or half-open connections are being restricted, you'll only be able to download from i believe 8 different people, which wont fare well for download speeds.
most people limit their uploads to a few kb/s so the trick is to download from as many people as possible