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I'm looking over what little I can find on the 8502, and from what I can see it appears to be a 2 MHz version of the 6510 with an extra I/O pin.
Much ado about the speed, but the Atari's 6502B's were doing 2 MHz years earlier, and the BBC Micro as well.
Were there any other timing/speed related changes that are not obvious?
The change in the MOS designator from 65xx to 85xx was due to the process change - original 65xx were NMOS process, while later 85xx changed to the new HMOS process. This allowed for better densities and lower power dissipation. A positive benefit being that the 85xx used less power even at higher speeds, with obvious benefits for cooling, PSU budget, and reliability. This likely explains why the 1MHz 85xx CPUs were chosen by Commodore for the Plus/4 and C16. Since the C128 needed to support a 2MHz mode, the obvious choice for Commodore by that time (1984) was the 8502 with the more advanced process technology.
So it's not that earlier NMOS CPUs could not be made to run at 2MHz. It's just that by the time Commodore made the C128 they had a better choice available in the 8502.
