sync vs async vs zfs

[Quartz]

>> 1) A SLOG's only purpose is to reduce fragmentation and increase
>> sync speed, correct? Re: speed, using a SLOG that's the same speed
>> as the other drives in a pool is mostly pointless, right?
>
> Correct. And I proved that on one of my servers in pre-prodcuction
> testing.

Tack-on question: would an identical-speed SLOG still speed up the pool 
by proxy simply by reducing IO load on the vdev(s) and/or reducing head 
travel on the drives?