From previous testing, we know we can achieve huge overclocks on the GTX 980Ti. The non-reference models already come overclocked slightly from factory, so the potential is limited slightly.
We can see here that Palit Super JetStream 980Ti can achieve a huge overclock of 1374MHz boost. The memory was left standard as it adds very little benefit.
3DMark Fire Strike
The overclock, while not massive, yielded a decent 7% performance increase.