I asked OWC about this 770MB/s speed cap and they said it's to do with the Ministack also driving the other output ports all from the one input. But if I went with NVMe I wouldn't mind a little speed increase. I'm still using HDDs for routine data backups and speed isn't an issue for my modest needs. I'm not sure I understand the point if it is limited to 770MB/s. I would hope that it would have a faster bus so that it mimics other M.2 enclosures with faster speeds. I do wish it had more USB Type A ports since I have a large collection of devices that want a Type A connection but it is easy enough to add a small USB Hub externally. They are still 5 times faster than an HDD so I suppose it’s all relative. saving a 100MB file to external SSD takes 0.14 seconds rather than the 0.04 seconds it takes to save to internal SSD) The exception is when backing up a large amount of files the slower speed significantly increases the time required. I have a couple of external USB3.1 SSD’s and I do find that 700MB/Sec is fast enough for my general use as I rarely notice they are 4 times slower than the Mini’s internal SSD.

NOT TB3/4 speeds) so that limits SSD drive performance to around 700MB/S. Review’s I’ve read indicate that the internal SSD drive (if installed) operates at USB3.1 speeds (I.e. I really like the form factor and included ports and drive bays. The STX costs $279 empty and you can add various OWC drives as well. Hopefully OWC has fixed the problem with this recent release.

But it went back to OWC because it totally killed my WiFi when stacked on top of the Mini and really degraded it when attached via a 3' cable. I should note that I once tried an earlier version MiniStack that I hooked up to a 2012 Mac Mini. Up to five Thunderbolt devices, three USB devices, and two displays. It has two internal drive bays: one 2.5/3.5' SATA SSD/HDD and the other is compatible with NVMe M.2. I don't know if this option will meet your particular needs but as a long-time Mac Mini fan it is interesting to me.

Usb hub for mac mini