I don't know, you can try an experiment: watch the Streamer_down_file and see when it reaches its final size. My broadband is only about 2.5Mbps anyway.
Update: I've just done the experiment, initially it looked like the data was being downloaded at about my broadband speed and I estimated Chicken Run would take 48 minutes. I got busy with something else and came back after 49 minutes - and it was done. Conclusion: we can rip iPlayer this way (no DRM!) at full network speed.
However, I don't see where it gets you from the slow-UI point of view, because you have to go through the same procedure whether you watch the download live or wait for it to have downloaded (you can't grab it until it has completely downloaded either). Unless you actually aim to save it (beyond the iPlayer window, or for repeated showings/archive), you might as well watch it at the time.