I have no clue. Maybe, cos` there is less that could fail (in a sense like you can get max 2 flat tires with a bicycle compared to max of 4 when using a car)
But even if it would be easyer to overclock whats the point since nearly every 040 optimised program will expect to find an FPU and crash so you have to stickt to the 020 version to be on the save side.