> 1. From my own experience, most people who use a ZIL for faster sync writes are wasting space. You only need five seconds of peak throughput (for a 10Gb connection that's ~5 GB). Create a pair of 25 GB partitions for the ZIL rather than dedicating entire devices.

You might benefit from the same technique for a "special" vdev... If you run an all spinners array, but the metadata is on a small partition, you might get better seek times during metadata scans, since everything will be closer together? And/or if you use raidz for your regular vdev, but mirror for the metadata, you'll have more capacity for metadata reads.