Heat is not the limit of overclocking.
You can have a CPU running at five degrees celcius and the frequency will still be limited. If you go pass this limit, the data will not process correctly because it is too frequent for the CPU's architecture.
Check it:
Remember the review on Tom's Hardware Guide where 5.x GHz was achieved? The CPU's temperature was way down in the minus, but they were still restricted to 5.x GHz, because when the frequency was pushed higher, the data processed became corrupt.