So, I just concluded stability testing of my Pi 3 B+. The memory is stable at 500 MHz with over_voltage_sdram_p=4 (and the other SDRAM voltages at default). That's a 0.075V over volt compared to the default 1.225V. I didn't actually test with just 0.025V or 0.05V extra voltage, so it's possible I could lower it a step or two, but I don't think it's worthwhile to spend time on for this setup.
530 MHz was not stable even with over_voltage_sdram_p=6, so there's no more SDRAM performance to extract from this particular sample.
Each board is different, so my overclocking endeavour might differ from yours. The take-away is that stability testing is hard and time consuming. Any semiconductor device is really only as good as the weakest performing transistor. So, to be sure that an overclock is stable, you need to exercise the complete device, which is in most cases not practical. The memory on my Pi is probably a good example of this. I stress tested it thoroughly with Quake 3, sysbench, memtester and linpack, only to find out that it crashed at the Emulation Station memory. It's quite likely that the RetroPie setup exercised memory addresses that my previous test setup did not (or exercised them in a different way).
So, happy tinkering and overclocking. But do expect some stability issues, even when you think you've tested everything thoroughly.
