Do you have any datasets with Sync set to Always? Or Sync set to Standard and have software that requires write sync?
If no, you don't need a SLOG. Even then you don't need one unless you have frequent power outages or are dealing with data you cannot bear to lose to a power outage. Your ZIL (i.e., your write cache)is in RAM. Your A2 cache (i.e, your read cache) is in RAM. Just get more RAM. There ain't nothing like seeing 128GB of RAM fill up over a 10GbE pipe on a freshly rebooted server as you send a file over.
The only thing you could probably speed up with those unless you have iSCSI drives or Sync write on on some datasets is marking them as either L2ARC (most folks don't need this, if your ARC hit rate is tanking it might be worth it but its not necessary) or metadata special vdevs (if they both fail you lose your whole pool).
How do I know?