I worked for a CPU company for 2 years (heading up the software team). The speed of the chip is determined by the manufacturer after it is fabricated. They run the samples at each possible speed and watch for errors. The manufacturer wants it to be rated at the fastest possible speed because those sell for more. It is in their interest to give it the fastest possible ranking.

So the ranking it has, that is the fastest speed at which the manufacturer is confident that it won't have any problems. When you speed it up you're moving it to a speed where it may have problems. Not will as the manufacturers are conservative, but well may.

For the small gain in performance - that's generally not worth it.

note: My experience with this is 20 years ago so how the determine chip speed may be different now.