Curious whether the AMDGPU OverDrive overclocking on Linux was actually working with the RX 580, I used the Phoronix Test Suite's integrated system sensor capabilities:
The GPU temperature was indeed higher when running with the 6% GPU overclock... As would be expected.
The system power usage was also slightly higher, thus looking like the 6% overclock was a success, albeit didn't do much for the Linux gaming results.
More monitoring results via this OpenBenchmarking.org result file.
That's where things stand today with the Radeon RX 580 overclocking under Linux with the AMDGPU driver. See yesterday's Radeon RX 580 Linux benchmarks for more performance numbers with a larger GPU comparison.