The CMOS rule of thumb is one gate is 4 transistors (eg a 2-input NAND). But the first Z80 was nmos, and so the equivalent count is harder to state. 3 transistors per 2 input nmos gate is a rough first order guess, but in nmos wide gates still need only a single load resistor, and designers sometimes used dynamic logic to save even more transistors.
Designs like the GreenArrays F18A core show that you can do dramatically more than the 6502 or Z80 did in a similar number of transistors. The J1 and J1A are free descendants. The MOStek and Zilog hackers were wizards but they were working under serious time and, in the Zilog case, compatibility constraints. We know enough to do better now.
