Undervolting,downclocking, even overclocking cannot possibly damage your chip.Overclocking may damage the VRM on the card (usually when already having the voltages increased), because you make the chip draw more power.But that's why they put a power limit to the cards.
So, you're safe if you don't mess with the core voltage.Undervolting and overclocking will only cause stability issues.There is absolutely no downside to downclocking other than lower performance of course .
If you want to increase the lifetime of your card(only applicable in my opinion if you are constantly running it at 70+ degrees C) first lower the power limit.You might encounter crashes or artefacts in games(graphical drawing errors, usually observable with the naked eye) then it means the chip cannot cope with it's lowered power available.It's best to get an intenssive graphical benchmark to observe this more easily(like unigine heaven or valley, or 3Dmark).But given that you have a HD7850 that's very close to impossible as this chip overclocks like a beast.
If everything is ok now you can further make the card run cooler if you lower the core voltage.This is where you will see the most significant results(tempwise).For that card you can get access to it with Asus Gpu Tweaker(I have a Sapphire HD7850 and that's the only app that can unlock voltage controll).Lower it slowly and keep testing untill you encounter stability issues(artefacts, crashes in the above benchmarks).
My advice is to first agree on a specific core frequency you are happy with(performance wise),set it(I think reference clock is 850mhz for hd7850 so that's a good value to settle to) and leave the memory as it is for now.Lower the core voltage untill the card starts to become unstable(this differs from chip to chip so you have to find it on your own),increase it a bit untill it runs perfectly ok(in the benchmarks) then increase it a little more to be absolutely sure you won't encounter any issues later.Note down the values(voltage,core and memory frequency).
Now for the memory frequency, since you don't have access to it's voltage, i reccomend going upward untill you again encounter stability problems, lower it untill there are none, then lower it again a bit to be sure .There is no point in downclocking memory if you can't lower it's voltage.
After you're done with all that you will have found a configuration(which is specific to your chip) of core voltage and memory frequency that gives you the previously wanted performance(agreed core frequency) with the lowest possible power consumption and working temperatures, squeezing every ounce of performance for the new (lower)power level you are settleing it into.
If you get Tech Powerup's Gpuz you have an option there called asic quality which will give you a general impression of how good the chip is(higher is better).To make a quick comparison, mine is at 82% and it runs at ~1075 Mhz for core voltage of 1.075v.The (excelent)cooler on my Sapphire card keeps it at maximum 50 degrees C in that config.I consider it a crime against humanity to go any lower than that .
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