First check the maximum trigger rate and maximum data rate for different sample lengths (for each channels):

sample length      Maximum rate   MB/s    CPU % (per thread)
16us               0.44kHz        231     20
6.4us              1.04kHz        214     20  
3.2us              1.95kHz        198     22   
1.6us              3.12kHz        160     26   
0.8us              4.88kHz        127     30   (what are these threads doing?)

Look at the code more.  See that there is a maximum size for the block transfer of 10kB.  Increase this to 130kB
(which is the maximum amount that this board can make per event).  Now find

sample length     Maximum rate       MB/s     CPU % (per thread)
16us              0.68kHz            346      7
6.4us             1.46kHz            300      10 
1.6us             3.6kHz             190      25

Good.  So for long samples we are actually slightly above the maximum transfer rate of 85MB/s*4 = 340MB/s

Tried writing out the data to disk at the maximum 345MB/s rate; the DAQ can't keep up.  Maximum rate was more
like 270MB/s.
I think the mlogger was actually fine.  But I think the write-to-disk speed of the harddrive could not keep up.
 So I think we are limited 
by hardware in that case.  We would need a large raid array to be able to write faster.