From Linux 4.11 and Mesa 17.1-dev, here are some overclocked RX 580 results with a 5% overclock and then a 6% overclock, the highest I was able to obtain. A few other cards from yesterday's results are in here for comparison.
The 5~6% overclock was yielding just a few extra frames in Dota 2 with Vulkan.
In more demanding Dota 2 tests, the overclock didn't appear to have any measurable impact, assuming this OverDrive functionality is working out fine.
In most tests, the performance ended up being nearly the same.