According to the info I have a 2x PCIe interface can deliver up to 500MB/sec: If I'm not wrong, therefore, the PCIe slot in a MacMini is for SSD only but not better than a SATA III (6.0 Gb/s) but surely better than a Fusion Drive system. Therefore replacing the traditional HD with a SSD like the Samsung EVO is enough and you can't get better performance. OWC's upgrades: As Apple doesn't go further than 256 GB. OWC does, but, as I already said not meant for speed improvement.
TNX to eveybody for help and clarifications. Regarding OWC, I believe they're just selling upgrades to existing PCIe SSDs as a PCIe slot is required: --- 1 PCIe solid state drive slot when purchased with a solid state or fusion drive, upgradeable up to 1.0TB. --- I believe I'll move to a faster standard SSD, maybe the Samsung 850 Evo Pro. Simon