What immediately jumps out at you is that linear read/writes
perform just as nicely or actually better with MQ than with the
old block layer.

What is amazing is that just a little randomness, such as the
find . > /dev/null immediately seems to visibly regress with MQ.
My best guess is that it is caused by the absence of the block
scheduler.

The block tree now has a basic deadline scheduler.  So for now I'd
suggest to kick off the discussion on all these prep patches,
and then retest with 4.11-rc1 or 2 that will have the scheduler.