There is an interesting article in CyclingNews (http://www.cyclingnews.com/features/friction-facts-bigger-pulleys-really-are-more-efficient) about derailleur pulley size. (Bigger is better and less drag.) The article also talked about chain tension (less equals less drag).
All this interesting but not rocket science. What is new is derailleur cages allowing idler pulleys up to 15 teeth! I do have concerns about relaxing the chain tension. The article talks of the small wattages saved with the lower tension. But doesn't less tension + more dropped chains on bikes ridden and raced in the real world?
At the end of the article was a line about how this relates to single speeds and track bikes. Apparently the writer never heard the advice I got many years ago. 1/2" of vertical play. Ie, no chain tension. (The fixie crowd here in Portland never got this lesson either. But the velodrome crowd do get it. "Sag" is visible on all their bikes.)