code size

ardnut:
If I'm working out how fast I can sample a obviously need to take the worst case figure, where did you get 104?

13/125000*1000000 ...13 clock cycles / 125000 clock cycles per second * 1000000 microseconds per second.

Is that an empirical result?

No.

What is the accuracy of the result when running async ?

The same as running synchronously. There is not a special asynchronous converter.

I doubt that they would have provided separate clocks and means to shut down the rest of the chip if it did not make a significant difference.

The point is to reduce the effect of noise not increase the accuracy.

Now the ADC is 10b with 2LSB of error in quiet mode. That's 9 bit accuracy ADC. If I run async what am I going to get?

The same.