Hi, I just saw some notification last year, I cant remember the detail now but I think it had something to do with the maximum frequency using a standard xtal (maybe)
and with my overclocking I wasnt using ADC, just bit blasting
I was using ATTiny13A/85 and ATMega328/1284 with 32Mhz clock oscillator
I think I found that you can use up to 24Mhz standard xtals with some of the ATMegas