Getting back to the OP's question....
Unfortunately, it is essentially impossible to set up any sort of study, independent or otherwise, to test the efficacy of DRM.
The only possible way would be to make an exact duplicate of the planet, and release one book in DRM form on one planet, then release the same book without DRM on the other.
Seriously, the only people who would fund such studies will have vested interests in one result or the other; same for those who opine and theorize on the topic. I wouldn't hold any more or less trust a study done by the Free Software Foundation any more than one done by the RIAA.