The AVRs are a 0,35 u 5Volt flash process and the ARMs are in a 0,18u 3.3V flash process. Can't really draw any conclusions between parts in processes that different.
Do not know any details why the downspec, but my guess is that you do not want to lose ANY parts due to speed constraint. Some flash AVR parts are known to run much faster than 24 Mhz.
You cannot run the flash at 66 MHz (30 ns cycle time) even in 0,18u. You therefore use well known techniques to increase bandwidth from slower memory. Typically these techniques rely on certain behaviour in the application.
Therefore you will not get the same number of MIPS from a flash part and a 32 bit zero waitstate SRAM part for all thinkable applications, but you can probably be very close.