The data that is currently displayed in the original post is pre patch 1.03. I've been farming a bit of these again lately (in patch 1.03) and while I do not have the data (yet) to conclude this, I have noticed the following:
Treasure Bandits appear to be dropping a lot of blacksmithing plans - I have found 6 plans so far from ~95 bandits while the other types haven't dropped any (~400 killed).
All four types seems to drop slightly more tomes/gems on average. The increase - if there is any - is very small and hence I won't conclude anything, as it may simply be due to the sample sizes.