RAID 1 with SSD and HDD

Yes you can use an SSD with an HDD with SoftRAID as RAID 1.
SoftRAID knows to only read from the SSD on reads. (faster)

Yes I can recommend this.

How does the SSD/HD RAID 1 combination affect overall system performance? It sounds like reads happen at SSD speed, but what happens with writes in terms of what the OS sees? Does SoftRAID treat the SSD as a "write cache" of sorts, or do we still need to enable write cache for that boost?

Writes are written to both disks at the same time, and so major write events will be slower, as they will be constrained to HDD speeds. Smaller writes should not be noticeable.

Write cache only applies to RAID 4/5 volumes.

SoftRAID does not "save" HDD writes for later, which is implied in your question, everything is in real time.

Gotcha, thanks. If you're looking for cool new features to add, it might be worth considering an option, something that might go alongside the write cache option, that would speed up writes to SSD+HD mirrors by writing to the SSD and "catching up" on the HD as time permits. Obviously that could result in (at least) a degraded mirror, but the performance boost might be worth that risk in many cases -- similar to the write cache, I would think. This would be a selling point, being able to enjoy SSD performance even when using a slower (& less expensive) HD in the mirror.

How does the SSD/HD RAID 1 combination affect overall system performance? It sounds like reads happen at SSD speed, but what happens with writes in terms of what the OS sees? Does SoftRAID treat the SSD as a "write cache" of sorts, or do we still need to enable write cache for that boost?

FYI, I have a SoftRAID RAID 1 Mirror using a Samsung 860 EVO 2TB SSD and a Toshiba DT01ACA200 2TB HDD. The drives are in a OWC Mercury Elite Pro Dual enclosure which is set in JBOD mode and connected via Thunderbolt 2 to a 2016 MacBook Pro.

The mirror volume is about 40% full. Current performance per Blackmagic Disk Speed Test is:
Read: ~500 MB/s
Write: ~165 MB/s

We actually hold a patent on this technology. ;-)

Look for it in a future version of SoftRAID that will actually implement this intelligently. It was more difficult to actually implement, but we think we have it now.

Of course real world, you will seemingly get faster performance, as the bottleneck is with continuous writes. Most writes are not streaming, so writes will appear much snappier in most cases.