I would put the voltage to max OV if you are trying to get above 1500mhz core.  1.200vcore does ok for 1490mhz and below. Also 3dmark draws the most power from the cards so that is why it is more likely to find a weakness that you may never see running heaven benchmark or playing a game.

Nope, Nope & Nope. 

I have the same problem you have and going with higher voltage doesn't resolve the crash in 3dmark (11). Driver crash occur pretty quickly and even more when its in the loading screen. In 3dmark we hit the power limit pretty fast and the max clock never get really high anyway. Here what I found so far and why we should stop looking at the MAX boost clock anyway.
 
For 3dmark the secret seems to be KBOOST. Disabling nvidia gpu boost and keeping the clock speed and voltage at maximum speed increase stability. You will still reach pwr limit when benchmarking so consider that higher clock may not give better result. When you hit the power limit your GPU will throttle and it will give you lower GPU clock then what you see advertised but Precision at idle.
 
Example,
 
3dmark 11
GPU: 1356mhz (OC offset +115mhz)
Memory: 2053mhz
Scrore: X7107

 
3dmark 11
GPU: 1376mhz (OC offset +135mhz)
Memory: 2003mhz
Scrore: X6977

 
3dmark 11
GPU: 1403mhz (OC offset +162mhz)
Memory: 2103mhz
Scrore: X7166

 
Keeping the memory clock will also help save power for the GPU and give you higher maximum clock speed when running 3dmark. Now if you keep boost enabled, the card get very unstable when switching clock speed. I'm not sure yet but it seems to be providing too little voltage on some clock speed when ramping up. My max stable GPU/Memory offset for my card was +80mhz/+400mhz with voltage at +87 and power target 125%. That set my GPU at 1321mhz and the typical clock when running game is 1476mhz. Its also stable 24/7. This is pretty much the sweet spot. Going to +90mhz make the card unstable when looping 3dmark for a while. Again the crashes occurs the most when switching clock speed when going from a demo to the other. The reason I up the the voltage to +87 is only to get higher clock speed under load. In my case it give around 30mhz extra.
 
My last test was with a custom bios to increase power limit,
3dmark 11
GPU: 1366mhz (OC offset +125mhz)
Memory: 2003mhz
Scrore: X7176

 
In this case it did help with BOOST disabled BUT it would still crash with KBOOST off for the same reasons I had with the normal bios. For day to day gaming and normal use I need the power saving so my maximum clock speed really is 1321mhz.
 
If someone at EVGA read this, please fix this boost crash problem. its really limiting our cards with boost enabled!