I have a bunch of R9 390s & 390x's, they are safe to overclock to 1100 1560 from 1040~1060 1500 ,, but not safe to overclock the memory to 1700~1724 which is their limit, even if they can handle it - shenanigans will soon happen after weeks and chip will die(live experience)
The 1100 1560 is actually a very decent boost believe it or not, as well as 1540 Vs 1500 gives more boost than 1700 Vs 1560
I have an core2duo E6400 lying around which is OCed to more than 50% boost on GHz on some decent mobo, with a stock cooler even and no issues in temps..
Had a 9800GT which outperformed every oced 9800GTX+ , only needed to flash it for extra values and to have fan speed always at 100% , but before you say anything - needed to have fan speed at 100% even on stock, because it artifacted on auto fan curve & froze then
Apparently I have golden chip of TR1920X which does 4.1GHz @ 1.32~1.344 (1.3125 in bios), it's a huge boost, but much more of a waste to watts usage since it's a power hungry chip on OC only.
Had unlucky i7 6700k chip, max it could do was 4.8@1.45 and 4.7@1.41 , but temps was not for everything, so I then done an alternative, 4.4GHz core & cache @ 1.35.. was pretty good & much colder
Nvidia GPUs generally can go sky high on OC which is good
Forgot to mention that I also have flashed timings on my R9 390/X's and that's a free 3% boost everywhere, though obviously due to a bit of a raise in FPS - temps raising a bit too, but it's a free extra if you already hit your limits on clocks
Ram timings, MHz, always a good boost to your system.. generally some values like 16 18 18 36 CR2 can go as low as 15 17 17 28~30 CR1 without changing the MHz and at the same amount of stability, and that's only main timings, timings overclocking experts also can adjust bunch of secondary timings for extra performance, and generally ram does not die today because of itself, it more oftenly does because something else has failed, like PSU or mobo
Enabling large memory pages in gpedit
Now combine all these small boosts together, CPU ghz,GPU,GPU timings(if available),ram MHz,ram timings ,memory pages, and you get yourself a much stronger machine than what you had, at very least 15%'s (at very least!) even if gpu didn't went far, yes, most of the boost will still be on your GPU side (unless your CPU is weakish for today's standards)
With all that said there is definitely some cons out there for some of the hardware, but not for all of them and in most cases it's worth it IF you know what you are doing and what to expect from what you are doing.
Overclock is totally worth it, try it, if you will be careful - you will break Nothing, just don't forget to monitor the temps, that's the same thing to do for both beginners & veterans overclockers