From what I have seen quoted the Sega CD CPU at 12.5Mhz should be 50-66% faster than the stock Genesis CPU. The Genesis CPU is rated at 1 MIPS, versus 25MIPS for just one 23Mhz SH-2 in the 32X. I had a rating for the Arm60 in the 3DO somewhere as well but I am not finding it at the moment.
There have also been a number of discussions about how much faster the Sega CD 68k is compared to the Genesis CPU around here and all of them have concluded that the difference was very significant, though not a generational leap. Combining the two, or at least using the two on separate tasks would made the Sega CD even more formidable.