you can and most of the time will lose warranty doing OC, and depending on the amount of OC given.
Overclocking is meant for performance increase, but the difference isn't huge, but is slightly noticeable.
Performance increase is hard to tell depending on the overclock but you could get a 5fps increase for a good OC on GPU
If you OC both expect around a 7+FPS increase, depending on the OC amount done also. Never OC to the point where the CPU/GPU is unstable (Overclocking to a damaging level), as that is not good in any case.