Do you mean overclocking a 4040, building the best 4040 with techniques available at the time, or how fast modern production could make a 4040?
Another example, the Chinese counterfeit Saelea logic analyzers use a 68000 core with a USB phy tacked on
I can imagine Intel to just have specified a narrow operating range where the device is guaranteed to work reliably within the specified voltage/power envelope. We might be surprised to see higher clock speeds if we play with the supply voltage for example.
I also had an idea to overclock i4040 more, but even 740kHz was hard to work with, because I had just 200 stm32 ticks per i4040 sub-cycle to do all I/O logic (RAM/ROM interface for i4040).