Try it out. Worst case scenario it won't work as you expect but you can't kill the card don't worry.

Yes. Unless the price of card is overpriced too much

Correct.
Or enter the address in the explorer.

You cant OC to +70% core or memory on these cards.
 For power for nvidia for example you can lower to 50%TDP and it works very good. Not sure about ATI how it would go. Never tried since on nvidia is easier.

They were limited back in December and they are still limited on most places like newegg or evga etc. And still they sell like hot bread
For some algorights yes Asic is a threat...also a big business for the sellers.
All good things come to an end ...eventually or transform.

Sorry for confusion. Pc memory does not matter.
The video card has its own memory and it can be Samsung micron or hynix.
Hynix has lowest hashrate for ethereum. But you can see the type in windows only by using gpuz program. I don't think you can see in Linux.
That will tell you whether it's worth the efforts to over clock much or not.
It is also possible you have not oveclocked anything thus the lower hashrate (if you have micron or Samsung memory)

If you switch to other coin not using the eth algorithm it would not matter the memory.

Can you show a pic how it look in Polaris editor? Open a fresh bios file without any changes.
Top right should have a drop down which shows memory types. By this you can figure which strap lines to edit for that memory. It is possible bios has 1 mem type. Although rare but possible. Other have 3 mem types like mine 570s. Etc.
Get the very latest polaris editor  github.com/jaschaknack.

Best bios is your bios

For elpida this should work . For above 1500 straps use below. no need to change anything else except use the strap on mem slots 1500 and above.

777000000000000022AA1C00315A5436A0550F15B68C1506004082007C041420CA8980A9020004C 01712262B612B3715


For over clock try initially core 1150  mem 1950 from Windows tool like msi afterburner or other.

Try
TDP 75,  mem +200mhz (real speed for linux) for starters. Core does not matter much for Eth.
But if you memory is Hynix, don't expect more than may be 17-18.

Or switch to equihash algorithm coin.
TDP 75, mem +200, core +100 for starting point.

Or if you have very cheap electricity don't touch TDP.

For dual mine
Rx580 are good with lbry lately on claymore but more power consumption
Nvidia lbry decred. No idea about pascal never tried.

Depends on memory type.
Samsung best
Micron good
Hynix lowest

Then we put memory OC into the end result.

The problem usually is not the voltage difference but the power frequency. 50hz vs 60hz.
If comparing Europe to Usa. Not sure what is the frequemcy on the US 220v.
Plus the lower watt limit on the kill-a-watt. I would not use the device on 220 as well but for example the TPlinks hs110 are OK if you want meter + smart.

But for accuracy I can say killawatt are good. I've compared with tester, with my ups and my tplink hs110s.

If history repeats and if coins stay where they are ...and if moon is 24 degrees to the left....n few weeks to a month should be better for video cards stocks...but then nvidia may be preparing for the next storm release ...so you never know.
Interesting is its easier to find RX AMD now.

If memory serves me right from the Signatum times
Core matters more.
...miner app too

Did you mod the bios on the 580s for starters? Elpida memory is not the greatest but you should be able to queeze 26-27 mh with proper setup +driver.
Forget the baseline without mod. Try it out on 1 card

Very first may be try the beta driver from August or the latest one and  set to compute mode.

by the way -  Unfortunately you have to forget about SIA now that there are ASICs.  Switch to something else for dual.

Sell , make profit, wait few months for things to settle down, try again with the nextgen GPUs.

the whole craziness came from the End Dec - early Jan baloon, the so called "futures" speculations/manipulations and the media. + people are greedy.
nowadays even grandma is mining in her kitchen and kids want for their birthday a gpu to mine
The funny part is most people say : ooh I mined and have profit 40$ today ,  have now 564$ ...etc......no you don`t. You got some crypto nothing unless you cash it out, pay dues and have in plain old cash.
So the perception is a bit fake as you see.

Yes - Coils. Happens on Nvidias usually.
Its usually with coins/miner software where there is large fluctuation of electricity up and down. when I look at my UPS stats - exmaple - goes from 450W to 650 for second, etc. Sometimes drops to 300+, sometimes for 700.    Don't know whether its the algo or the miner software that is so not efficient.
I get this on zcoin and used to have it on signatum with some of the crappy miners back in Aug. Never tried X17 but I can bet its the same thing.
If you get very near to the GPU, like almost touch it with your ear you can get the noise clearly.  The funny part is the PSU could also eventually get this if its abused and not of solid quality.

To avoid all that, switch algo  Whether its harmful for extended periods - I don't know. Never tried more than few days.

Sometimes it makes more on dual on Nvidias. Personal choices I guess.

About MPH - I've noticed lower amounts recently as well. No idea why.

Yes and No at the same time. Depends on multiple conditions.
Main factor is the niceha** disruption in the normal organic growth from what I've seen lately.  If we do not take into account smaller pools who do not find blocks that often.

Example.

you are supposed to earn 1.9-2 coins today, slow organic growth of difficulty and hashrate taken into account.
Niceh** user comes in, adds large hashrate for few hours or half a day, your coins drop to 1-1.5 per day for few hours or more. Difficulty also jumps.
This repeats lately quite often for cryptonight coins.   Then you have the number of blocks founds per day, etc, etc.

So in short today you may get 2 coins, tomorrow 1 coin, the next day who knows.
 Either stick with a coin you like and do not care about what comes in or auto-jump on a daily basis. Up to you. Try to find a pool which has at least 10% of the network hashrate.

If you believe that miners would sell current generation cards    Gamers would probably sell - yes. but limited quantity.
Usually what I've seen before is miners just buy more cards. The moment they sell (speaking large scale) is when the card is totally useless for mining. Which would take at least 1 more year for the current generations video cards.   I think the future is more and more video cards put into mining, bigger difficulty, higher hashrates, profits returning where they should be, not like the end of Dec craziness and may be more NiceH*** clones shaking the hashrates