I've had my MSI GTX 970 Gaming 4 since September now and I've had plenty of time testing it and overclocking as well as flashing my own custom BIOS's. I have to say i'm very dubious about people that claim 1550mhz 8000mhz is truely stable with no voltage or TDP limit bump. I found that running benchmarks and gaming for short periods or on certain games this was achieveable by simply increasing tdp to 110% but under real stress it would always crash or throttle back the GPU.
In the end I modified the entire voltage scale on the cards BIOS, topping out at 2.81v @ 1550mhz / 8000mhz with a TDP max limit of 300w & i have seen it actually reach 260w several times. This allows the card to run rock solid stable for hours on end in anything I've thrown at it and it never scales back the gpu clock. FC4, BF4, valley bench and heaven all run perfectly with no artifacts, hangs or crashes.
Here are my scores to prove results: