Amd have form for pushing out a lot of performance through drivers. Raja has said RX Vega will be faster. We could be talking a few % or 20% as remember how big a leap some games got through miracle drivers with the 7970. It pretty much went from gtx680 performance to a commanding lead. Could they be putting the final touches on a performance driver that was not ready for the FE release nobody knows bar them. Nvidia are the more professional outfit and just about get the full performance at release where as AMD seem to keep extracting more and more through time.
Drivers won't give you a 20% performance increase. In an ideal world it would, but in reality it won't.