It doesn't work like that exactly. A cool chip will be stable at a higher clock speed than a hot chip. If you run the chip at almost the top of the thermal envelope, you'll be lucky to get it to run at its normal clock speed.
With regards to overclocking, remember that a majority of all of the m68040s made in the early and mid 1990s were all the same mask meaning that a 20 or 25 MHz marked chip is identical to a 40 MHz marked chip. I have overclocked dozens of 25 MHz CPUs to 40 MHz without ever a problem. Of course doing so requires adding a heat sink if there isn't already one.
I have a bunch of QuadDoublers which run at 50 MHz and they all work fine. They have heat sinks and some don't even have fans - just some air movement from the power supply fan is enough.
The biggest problem I've seen with overclocking m68040s is with the clock generator. Replacing a MC88920 with a faster-rated MC88916 is usually a good solution, but only if you're having stability problems.
Some of the Amiga accelerators which can take either an m68040 or an m68060 have completely different clock generators which can run much faster. In that case, if you find a new mask m68040, you should be able to run your CPU much faster - perhaps 66 MHz or more. One of these days I'll find one and give it a try.
