Well first off I can tell you... You dont have 100mb Dl speed.
Your LAN (2 computers in your house) can go up too 100mbps, and thats with an ethernet cable.
On the wan side i would say your connection speed is PROBABLY around 1.5mbps. BUT in reality you get aroun 800kbps.(download)
I would say your upload is around half of that.
If you get a torrent downloading about 50-60kb, consider yourself lucky.
About the proportional speeds thing. Think of your donloading and uploading as a pipe with water flowing to it. Downloading the water flows one way, uploading the water flows the other way. Torrents require you to upload a little, and if you go more than that you are cutting down on your max download.
You could upload at 100% and download at 0%... (which would be bad)