Solution
The reason that the rate is not 250 kS/s/channel is because the update rate is dictated by how fast the data is being streamed from the FIFO to the output channels - there isn't a linear correlation between the sample per channel rate and the maximum update rate as with Analog Input measurements. For 9-32 channels the sample rate maximum for all channels will be 350kS/s. For more information on this, the 'Maximum Update Rate' section on the
MIO Specifications document might help.