this thread motivated me to do a little research on the possibility of upgrading the base Mac mini 2014 with a PCIe SSD.
From what I have read so far, it should be possibile even with Mac minis that originally did not come with such a drive, all the connections seem to be there.
owc even sells an upgrade kit for that specific Mac mini model containing everything from the drive itself (the aura series) to tools, cables and screws.
one very useful thread has already been posted above:
In searching the web a month ago, I wasn't able to find articles on using a standard M.2 PCIe NVMe SSD in the 2014 Mini. I was going to order a Toshiba/OCZ RD400 for some yet-to-be-defined future use NOT in the 2014 Mini (it's a good PCIe SSD that Toshiba will be discontinuing and the 256GB...
forums.macrumors.com
I also found an instructional video on how to actually install/upgrade the drive:
In this video, we show you how to upgrade or replace the PCIe SSD in a Late 2014 Mac mini with a new one, such as the OWC Aura Pro X2 or the OWC Aura N.
blog.macsales.com
maybe, sitting at home day in and day out ..., I might give it a try. the procedure seems to be straight forward and much easier and quicker than replacing the HDD with a SATA HDD for which you need to dismantle the whole mini.
currently, I am booting my mini from an external usb3 drive which is fine, much better than the internal HDD anyway. usb3 speeds are ok, but better suited for large file transfers.