So I just bought a new cMP, and I’m having a hard time finding the answer I need. What’s the most efficient way to use an SSD in the cMP? It seems that you can get much faster speeds using a PCIe SSD than one in the disk slots. For a small SSD to boot macOS off of what would you recommend? Im coming from a retina MacBook Pro which has decent read/wrote speeds. So I wouldn’t want to be much slower than that.
For booting macOS, any SATA SSD can do the job (trust me, all you need is just plug it into the optical bay). The 4k random read figure usually about 35MB/s, way below the SATA 2 limitation. That's why you won't get faster boot time, apps loading time, etc by upgrade from SATA 2 to SATA 3 (or even PCIe SSD).
It's doesn't mean faster SSD is useless, but may be only save you few seconds in total for a whole day (only consider OS operation). Is it worth? That's very personal.
I have a PCIe SATA 3 card, but the reason I use it mainly because it give my 2 extra SATA ports. In fact, one of them is only connected to a 2.5" HDD, not even a SSD.
I tried both SATA 2 and SATA 3 connection. The gain? Practically zero (again, only consider OS operation).
However, if you deal with lots of large files loading / copying / cloning (without APFS) or (un)zipping, then PCIe SSD is the way to go.
The most cost effective way to boot macOS from SSD on the cMP is SATA SSD without any PCIe card. And this give you virtually the same performance as any other solution.
I don't know how you use your Mac. But this is what my Mac did in the last 30 days. 99% of the time don't even need 25MB/s.
And in the last 24 hours, the highest loading is about 125MB/s. This is not the real peak, because there may be a split second that the SSD really working at above 300MB/s, but just can't be displayed in the chart. And that's why I say a faster SSD may save you few seconds per day (for normal OS operation).