Você deve ter mais memória RAM no sistema. Ponteiros para L2ARC precisam ser mantidos na RAM (ARC), então eu acho que você precisaria de cerca de 4GB ou 6GB de RAM para utilizar melhor os ~ 60GB de L2ARC que você tem disponível.
Isto é de um tópico recente na lista do ZFS:
L2ARC is "secondary" ARC. ZFS attempts to cache all reads in the ARC
(Adaptive Read Cache) - should it find that it doesn't have enough space
in the ARC (which is RAM-resident), it will evict some data over to the
L2ARC (which in turn will simply dump the least-recently-used data when
it runs out of space). Remember, however, every time something gets
written to the L2ARC, a little bit of space is taken up in the ARC
itself (a pointer to the L2ARC entry needs to be kept in ARC). So, it's
not possible to have a giant L2ARC and tiny ARC. As a rule of thumb, I
try not to have my L2ARC exceed my main RAM by more than 10-15x (with
really bigMem machines, I'm a bit looser and allow 20-25x or so, but
still...). So, if you are thinking of getting a 160GB SSD, it would be
wise to go for at minimum 8GB of RAM. Once again, the amount of ARC
space reserved for a L2ARC entry is fixed, and independent of the actual
block size stored in L2ARC. The jist of this is that tiny files eat up
a disproportionate amount of systems resources for their size (smaller
size = larger % overhead vis-a-vis large files).