It's a bit nuts to suggest that the decision to add the Z80 was in any way influenced by the fact that, decades down the line, it runs much faster than the 6510 did in the mid-'80s. At the time, the usual clock range for the Z80 was 2-4MHz (I don't know whether they'd got up to 8MHz parts at this point, but if they did they certainly weren't common.) And that's just clock speed, which is a notoriously inaccurate indicator of performance between different architecures. The Z80 is a fairly capable CPU, to be sure, but it takes some 2-5 times more cycles per instruction than the 6502; then again, it can do a bit more per instruction. It mostly comes down to code quality. The reason so many of Commodore's competitors used the Z80 is because Zilog, unlike MOS, wasn't owned by Commodore.
(Also, the 6502 does not use multiple clock signals.)
In any case, psxphill is right; CP/M was pretty nearly dead by 1985, having been supplanted for business use by MS-DOS, and never really catching on with most Z80-based home computers the way it did with the hobbyist micros of the late '70s.
