The adc prescaler is set to 128, resulting in 16 MHz / 128 = 125 KHz on the Arduino. A 1MHz clock will result in 7,8kHz for the ADC...
And about OSCCAL: if you run the Arduino at 5V at 25 degree Celsius, then it should work with 9600baud. I tested it on attiny84 chips and only a few of them make problems. But if temperature changes or voltage drops, the internal clock might run too fast (or too slow) and then falling back to 4800baud should solve the problem.