I don't know but Federer was driving his backhand pretty well at this year's French Open. Did you see his 2nd round match against Falla? Falla was attacking Federer's backhand relentlessly with his lefty crosscourt forehands and Federer was nailing his backhands back just as hard and with interest. Federer's backhand was very consistent and it never broke down.
This is one reason it was such a shame that he didn't get the chance to use his improved, more confident backhand against that other lefty we all know and love at this year's RG.
Somehow, inexplicably, Federer has improved his backhand drive this year. Look at his backhands at the Australian Open, and compare it to the same shot in any other of his matches vs Murray and you will see a huge difference.