If I don't jinx myself with this post, it looks like I too will get an extra 600 Mhz outta my R7 1700. Started with trying a setting I found online for 3800Mhz and 1.3 volts. That crashed as soon as Prime95 started, so I just kept dropping it down until Prime95 runs and so far it's been 10 minutes. Gonna let it run for an hour to call it stable though. Stock AMD Wraith air cooler, and temps maxing out at 80C.
My BIOS doesn't have a drop list of voltages to choose from, but I might try dropping the voltage down if 3.6Ghz runs stable. Not sure if it needs 1.3 or not.
Edit: Damn it!! Just shy of the hour mark it crashed. This kinda sucks... not getting shit outta this OC! but could be bad voltages?