Studies have been done, and it's largely dependent on the amount of cross-chaining as well as tensioner drag and cog size. The best scenario is to run a front / rear combination with straight chainline, a large rear cog and a loose tensioner. Cheap derailleur tensioner pulleys with bushings instead of bearings can cause significant drag. That's why using a tensioner with a single speed setup largely negates the benefit of eliminating the derailleur.