I just looked at that OWC M24 Drive enclosure more closely and it seems to be an awful cheap design that only routes one PCIe lane to each M.2 slot. So will only get top performance with all four M.2 drives in the enclosure and soft raid. Pathetic. I'd assumed the device used a PCIe MUX to do this properly. Apparently not.
Here is a enclosure with a PCIe switch and active cooling.
https://www.amazon.com/Netstor-Thund.../dp/B00TVIOPXK This is a really interesting product class to watch.
I've got questions into Sonnet support about how their 4x4 card works in an expansion chassis... but by then you are talking a big large box for a few tiny M.2 cards.