Hi Oliver,
No experience with iMacPro, but quite a lot with the regular iMac.
I run about five edit suites of Premiere, all iMac Retina 5K connecting through a 10Gbe network to a GRACK 12 storage. I use Sonnet Twin 10G adapters. Running on MacOS 10.13.6, using AFP protocol.
Two of them have almost never had issues, they are 2015 models (TB 2) connected to a Thunderbolt 2 Sonnet adapter. The other three are 2017 models (TB 3) connecting through a newer model Thunderbolt 3 Sonnet Twin 10G. On those three TB 3 I’ve experienced several issues over the last year. Often times no connection showing after a fresh boot, the Sonnet adapters not showing connection lights, etc… Trial and error, reconfiguring the cables would eventually solve the issue; but almost randomly it seemed. Have tried things like fixed IP’s and re-installing drivers to no success.
I have sent the TB 3 adapters out for repair and hooked up the 2017 iMacs with some old Sonnet TB 2 adapters I still had lying around. So far smooth sailing for a month or two (despite having to connect these using a TB2-TB3 adapter now). I havent tried using the repaired TB 3 Sonnet yet (got them returned about a week ago). The speeds are consistent I believe all year round: about 500-600 MB/s on the blackmagic disk speed test.
One more thing: in the past I have had poor experiences using SMB protocol, also with random disconnects or inconsistent speed behaviour. This was on MacOS 10.12, where apparently there were bugs with SMB signing (you even had to trick the service using terminal). I haven’t used SMB ever since: AFP seems stable, the GRACK supports it and the speeds are sufficient for our work.
Hope my case study teaches you something and best of luck!
Merlin