Bah, every time I think I've got a handle on ZFS I learn something new that tells me I haven't configured my system properly. I've got 2 mirrored vdevs and a 120GB SSD as L2ARC - I'm not sure I'd get much more of a benefit from SLOG, although it might speed up some jails. CIFS transfers are going at 800 - 900 megabits, and moving to 10GBe isn't feasible yet ;).
OT: Anyone seen behaviour like this? https://lists.freebsd.org/pipermail/freebsd-net/2016-Februar...
Just expect a hit on performance if your workload leans heavily on the L2ARC. Also, in my experience, L2ARC is a lot less useful than a lot of people think; doing some workload characterisation is an interesting and informative exercise.
As an aside: an SLOG is usually useful. Unless your workload is effectively read-only, a SLOG can make a pool perform close to SSD levels for most workloads on the cheap. Just be careful to get a quality SSD for the SLOG.