Right. I am well aware of RAM requirements of ZFS deduplication which is why I'm not using it and not recommending it. With this out of the way, lets talk about SLOG.
As mandated by POSIX, ZFS will by default complete all synchronous (as requested by caller) writes before returning to caller. Also, any metadata changes in ZFS are performed synchronously. Many filesystems use journal for this purpose, ZFS uses ZIL, i.e. ZFS Intent Log which is either carved from storage space of your volume, or placed on dedicated volume depending on 1) presence of "log" device 2) option logbias . Also, ZIL can be explicitly disabled (thus making filesystem behaviour for synchronous writes non-compliant with POSIX)
Now, assuming that ZFS setup has not been "optimized" either by "logbias=throughput" or "sync=disabled", there is big benefit from having dedicated log device with low latency, because that allows all synchronous writes to complete after intent has been written to such dedicated device (as opposed to writing to data volume with large latency). Looking at latency figures, ZeusRAM is up to 0.023ms and Intel P3700 is around 0.02ms (however, ZeusRAM capacity is only 8GB and P3700 starts at 400GB - which leaves lots of space for other purposes such as L2ARC, however we do not know max latency of P3700 only average). This number should be compared against latency of spinning rust storage (or whatever is used for main data volume) which typically would be somewhere between 2ms - 12ms depending on specific HDD in use. This means that synchronous writes would complete much, much faster if dedicated log device such as ZeusRAM or Intel P3700 was used. Normally this could significantly boost IOPS number.
However, what we do not know is whether 1) Maxta does actually use synchronous writes 2) its underlying ZFS ZOLVs are not "optimized" to avoid using ZIL. It would be interesting to learn this.