You can always try it on your own board by first changing the soldered crystal feet into socket type. Then plug in different frequency rate of x'tal in it and see that if your board still works.

I did it once on a 16MHz 8052, and ran up to 20MHz. It died at 22MHz. A 25% boost is not too bad for me.

I don't think any cooling methods is useful. Because a 8051 generally starts up in cold when it is powered up. If it does not work then, it won't work later.

However, I can only imagine that overclocking on a 40MHz 8051 will be harder.

Regards,
Oliver