L2ARC settings are tricky.

Too low and they tank l2arc performance.
Too high and they tank pool performance.

However the autotuner defaults are for a fusion i/o card
and that is almost never right.

Ticket: #6614
Add a second certificate, for nightly builds.
The reality is the addition of an L2ARC device always sacrifices some
write performance for read performance.

These tunings seems to be a best effort at balancing write performance
with improved cache read performance.

Ticket: #6614