The 970 is locked to a voltage limit of around 1.3V. To push to that, as already said, you have to edit the BIOS and remove the restrictions—which are there for reasons other than to stop 970 owners beating 980 owners.
To add my own 2c to that, I don't recommend flashing a custom BIOS without first testing what kind of scaling your silicon is good for. The reason I say this, many 970's stop scaling well after 1500Mhz. They just stop gaining any tangible performance. Some cards keep going with increased benchmark scores, while many others stop. I edited my BIOS. I removed minor throttling by stabilising and increasing voltages. It improved my Fire Strike and Valley scores marginally, and it did not improve stability. I was recommended to do this by those with very little experience. To them, flashing a BIOS was just the thing to did. 'Oh, you're throttling. You best edit the BIOS'. 'Oh, your score is not as high as mine. You best check you're not throttling. I know you are and I know that's why your scores suck. So you best edit your BIOS'. This is not good advice.
Also, regarding the point about performance issues, there seem to be a high amount of variances in scores with Maxwell. For instance, my 1560/7800Mhz G1 Gaming 970 scored significantly less than another 970 at 1500/8000Mhz. I've seen folks come in with overclocks of 1620/8200Mhz, but with benchmark scores that don't reflect the overclock. It seems that frequency—and I believe Kingpin has recently confirmed this—is not everything in Maxwell.
In general, pushing a 970 beyond 1520/8000Mhz is a little silly. If yours scales well, I guess go for it. But if you're hitting the ceiling of your silicon, no matter what frequency you put on it or how much voltage you throw at it, it'll still be hitting the same scores. I personally recommend you find your max stable overclock on the stock BIOS and accept it. 1500/8000Mhz is the sweet spot. Anything more is just gravy.