Well, like everything else: Is bandwidth your primary problem, or is CPU? Whenever I run apt now, download time is nearly nil but installation time is forever. Increasing installation time (and complexity, and disk space and cacheability on the mirrors) for saving some download time is unlikely to be a good tradeoff. Of course, if you are stuck with severely metered 2G somewhere in the woods, you may very well think differently. :-)

Similarly, I've turned off pdiffs everywhere; it just isn't worth it.