Also, if you realy want 'ultimate speed' you have to look at actual Versions.
close Z80 relatives (like 180) are available up to 32 MHz (I have seen only
20s, but well) while actual versions of the classic 6502 and 65816 is able
to do 14 MHz. Now take in account that a clock ratio of 1:3 to 1:4 for 6502
The 65816 can go up to 20Mhz. It is used in the SuperCPU for the 64 and
128.
vs. Z80 (*) is a good rule of thump, so a 14 MHz 6502
equals a 42 to 56 MHz
Z80. Quite a racer ...
Cheers,
Bryan Pope