Hi !
I'm very interested in fast analog ports on arduino too.
So i tried some speed tests on this instruction v=analogread(pin); (v and pin ar ints).
Prescaler Maximum sampling frequency
16 62.5 kHz
32 33.2 kHz
64 17.8 kHz
128 8.9 kHz
To get the maximum precision, i would take just the sampling speed i need, not more. I didnt test at prescaler lower than 16 because of the datasheet 1MHz limit for the DAC speed.