RE: Downloading : how dangerous is it?

No, because they can’t detect that accurately without conflicting with user’s privacy… E.g. Skynet (Belgian provider) tells they can monitor how much you transfer through ADSL. That’s not (yet) true. Maybe they can aproximate, but nothing more. Believe me. And technically, you can hardly slow down one connection.

What would be possible is that, due to a high number of users on a branch of the network, a longer distance to the next “relay”, or some damage to the carrier, a connection is slower.

Add to that some performance related problems that can arise in your PC.