If you read up on L2ARCs your L2ARC shouldn't exceed 5x your ARC. You are obviously at something like 10x your ARC size because your ARC is almost certainly not more than 10 or 11GB in a "best case" situation.
I know you've heard this from me at least twice, but 16GB of RAM is NOT enough RAM for what you are trying to do. One of the reasons is because L2ARC uses ARC for it's index, which means you need even more RAM to not hurt performance. We've had lots and lots of users that wrongfully assume this is like windows and more hardware is always better. In fact, with an already stressed ARC because of it's small size adding an L2ARC can actually decrease performance. Yes, I just said you can add more hardware and see performance decrease.
Much of what you are hashing out in this thread is clearly explained in my noobie guide. Your problems have been documented by dozens of users before you that aren't happy with the "add more than 32GB of RAM" mantra that we give here, but that's the harsh reality. You are welcome to continue down this path, but just like the dozens of people before you, you are spinning tires on problems that aren't going to be solved by spinning the tires again. At some point time is money and it's just not worth your time. I've literally seen a few users spend months trying to get extra performance out of insufficient hardware. Some of the guys have spent so much time that they realized that if they had been paid minimum wage for the hours they had spent trying to make their existing hardware work they could have used those wages to pay for the appropriate hardware more than once over.
And FYI, there's another thread around here debating the whole ISCSI vs NFS thing that is actively being used, so you might want to read up on that thread. It's not a whole lot different than this thread, but you might want to read it just to see what has been said there as it's been slightly different than this thread.