I have put together a minimal system to test everything would work the way i designed it, and I had a school project that involved doing an experiment. So i decided to test how well the Z80 overclocked against my home PC. I found the results surprising since the Z80 maxed out its clock multiplier chip at 32 MHz, and my PC crashed at 4.3 GHz. This may not seem surprising until I worked out the percentages. The Z80 overcooked 320% faster than it was designed to go and my PC only hit up to like 122%. If the clock multiplier wasn't limited to x8 on a 4MHz base clock, I'm sure the Z80 could go faster.
Below is the Z80 test board I made: