Each core clock on any GPU will be either as advertised or very close. Although with Nvidia's GPU Boost 3.0, it will jump much higher than the advertised core clock. This is usually up to the silicon lottery, but buying a higher end card (like the one you proposed) can help as these are usually chips that are binned together to be of a higher quality than others. For Benchmarks, there are plenty of applications. The 2 that I use are 3d Mark (any variant of Fire Strike or use Time Spy) and the openGL mode of Cinebench r15.