And more for fellow storage geeks... that find this thread.
I heard back from the helpful folks at Sonnet. It seems their Sonnet M.2 4x4 PCIe cards are PCIe switch equipped so even when you don't have a 16 lanes of PCIe to feed that card, like when you only have 4 lanes inside a Thunderbolt 3 expansion chassis that you can get full bandwidth to any one M.2 card unlike in that OWC Express 4M2 drive enclosure. That's really great design.
Oh so close yet so...
Unfortunately the Sonnet M2 4x4 card is really intended for 16 lane PCIe slots and is physically large and only fits within their largest Thunderbolt 3 expansion chassis. But it seems that users have used the 4x4 SSD card in the same chassis as HDX cards... and I'm willing to bet that's not "officially supported" by Avid or Sonnet but it's interesting to know.
Because of its size the 4x4 card will only work in the following Sonnet chassis.
https://www.sonnetstore.com/collecti...d-thunderbolt3
https://www.sonnetstore.com/collecti...r-thunderbolt3
https://www.sonnetstore.com/collecti...akaway-box-550
Which makes it kinda impractical for most folks with all that size... but I have a need coming up that means I might get to try one out in my breakaway GPU box for non DAW use. (no promises).