I've not used a Bit Torrent proxy myself. They basically act as an anonymous proxy server, masking your location and IP, and just revealing the company server IPs. Not sure about slowing a torrent down, although most private trackers ban their use because you can't connect to a ratio based tracker through them properly.

Do you use Peerblock? It knows and blocks all IPs trying to look at your connection whilst torrents are downloading/seeding and stops companies logging your activity.

If you are really worried about your download content being illegal then I suggest you use Usenet instead. You can get most things on there, much faster than torrents, and no seeding or uploading, so no sharing issues. Plus it's cheaper than torrent proxy server subscriptions. I use NewsDemon and download using Newsbin Pro. http://www.newsdemon.com/