Another bit of info that might be of use: Secure Boot also seems to disable TRIM on external drives -

Setting the new Secure Boot setting to No Security seems to have fixed it for me: Same here, I can now boot my iMac Pro with my Thunderbay connect...

Not yet. We are working on getting an iMac Pro in, as we have been unable to figure this out remotely. Apple has ignored our bug report so far. ...

Cool thanks. Follow up question: I've currently got a single 6TB volume on the array (i.e. 100% of each 2TB disc is used for the 1 volume). I'm lo...

Are you planning to have an APFS-compatible version of SoftRaid ready to go when High Sierra is released, or will it come out later?

Sorry no. We understand that MacDrive is going to try to do a RAID 4/5 in the future and we will help them if they have any problems/questions. ...