AMD released an updated graphics driver for iMacs with Retina (2014+), MacBook Pros (2015+) and Mac Pros (2013+). You can download it via AMD’s Download Portal (“Manually Select Your Driver”).
The driver is dated February 17th although released beginning of April. Driver version is 15.301.2101 which is obviously behind Crimson 16.4.1’s version 16.15.2211 driver.
If you don’t wanna use a modded driver or if the modded driver does not work for you stick with the official one.