Yes, very curious indeed. I would expect the mirrored configuration to perform better not slower.
Most of my resources who could look at this are tied up in meetings all week but I have a few
1. What kind of write workload is this? Can you provide your test dd syntax?
2. Do you see any SSD related errors in /var/adm/messages?
3. Any unusual SSD temp readings?
Does even on a simple (1 vdev) 2-way mirror the problem happen?
What if you add a dedicated SSD for ZIL?