Just tried the new AMD drivers (Crimson ReLive Edition 17.11.1) and saw a 20% drop off in speed. Went back to the blockchain drivers (23rd Aug) and speed back up. Not sure if this is isolated to my rig of RX480s but noticed drop was on both 8GB and 4GB models.
Propably you forgot enable compute mode in AMD control panel.
Still i advice you to stay where you are if your system is stable!