It seems that the issues I reported on back in January have been resolved in a new Steam Client Beta. I have had download speeds go from 25k/sec tops to around 100k/sec.

A Steam client beta is now available. To access it, go to File -> Settings and on the Account tab choose “Steam Client Candidate” in the Beta participation section. The specific changes include:

- Fixed downloads becoming paused
- Improved download speeds
- Decreased application launch time
- Fixed URL links in chats to not include trailing punctuation as part of the link
- Changed group event notifications to open at most 8 dialogs

Steam Forums.