If I understand your setup and your benchmark correctly, you are saying
you have achieved 11 megabytes / s performance out of a volume of 24
RAIDZ2 drives split into two parts (so it's like RAID 60). Doesn't this
number seem extremely low to you, considering that (if recent models)
each of your drives can probably pull at least 70 MB/s?
It is also quite strange that a linear read file gets stored in L2ARC,
which usually holds random accessed data.
Maybe it is very fragmented on disks.
L2ARC with MLC drives usually is much slower than modern disks when
it comes to linear reads.
Are there any facts backup your assumption that data is really
read from memory, SSD, disk in the named cases?
E.g. by ARC/L2ARC and IO statistics.