Include 16MB-blocksize read/write case

As per suggestion by Dave Taylor, this shows that performance actually
decreases again at a certain buffer size:

    read+write bs           204ms      249ms      264ms
    read+write 4bs          206ms      420ms      793ms  (+ 1.0%)
    read+write 16bs         223ms      305ms      324ms  (+ 9.3%)
    read+write 256bs        251ms      259ms      259ms  (+ 23.0%)
    read+write 4096bs       287ms      408ms      457ms  (+ 40.7%)
    read+write 1k           347ms      439ms      472ms  (+ 70.1%)