I tried a Mega and five is the limit. The problem is that each ADC read takes 110 microseconds so there isn't enough time left in the SD thread to format the data.
You don't lose much accuracy by speeding up the ADC clock so that could help.
Finally you could trigger the ADC with a timer compare and read it in an ISR. You could write binary...
But all of this is way beyond an example for NilRTOS. Right now I want to improve all the RTOS ports.
I was thinking about porting NilRTOS to ARM boards but I decided ChibiOS/RT is a better fit.