Greetings,
 
so i tried to overclock my 1070 SC, following this video guide: 
i pumped power limit to 112%, core voltage to +100%, made a custom fan curve. And i couldn't go further that +75/350 (core/mem), which i found kinda disappointing since i saw many reviews and users getting 100/500+
 
According to the video, you should avoid hit the power limit.
I didnt checked that when overclocking, so i monitored power and voltage limit during a Overwatch match (Max Settings, 141% Render Scale)
And got this graphs:
 
(put https) puu.sh/rUxvX/58c24d3dbb.png 
 
As you can see, there is alot of power and voltage limit hits. This overclock was stable, i had 0 artifacts, crashes, freezes or any other problem.
Then i checked Overwatch (Max,141%) and heaven benchmark with stock configuration.
 
 (put https) puu.sh/rUxwp/da3669fd42.jpg
 (put https) puu.sh/rUxwy/4cac6531aa.png
 
To my surprise, the card hits power and voltage limits even on stock.
Later i was playing Doom (Maxed) with overclock and i noticed that doom doesnt hit power limit and when that happens the voltage and clock is always maxed (2000hz/1.081v).
That was a strange result to me, why Doom doesn't hit power limit with overclock and Overwatch does with stock clock.
Then i noticed that Doom has a low GPU usage, about 40~50%, while Overwatch has 80-90%. Consequently Doom has a lower Power usage.
I lowered Overwatch GPU usage by reducing render scale to 100%.
 
 (put https) puu.sh/rUxvW/4637260499.png
 
And it has similar results to Doom.
 
It is hitting power limit really a problem? if so, why the card hit power limit with stock clocks? Could that explain why i get a low overclock?

Hitting any limits isn't a problem as long as your card is running at advertised clocks when using stock clocks.

To increase the power limit you would need to run a modified vbios. Currently that isn't possible.

I would never ever put a 1070 in SLI, you will be running heavily into bus width limitations.
 
Consider this: a 780ti has 320GB/s bandwidth, the 1070 has 256GB/s with at least 40% more powerful GPU core. Now put two of those cores in SLI and you're still pushing all that through a measly 256GB/s bandwidth. At 1080p you will never need the horsepower, and at 1440p and up you'll not be able to use it proper due to low bandwidth. Let alone 4K... Pascal's delta compression won't be enough to mitigate that either.