According to Sonnet Tech support:
"Feedback from our engineering dept......
We're going to have to investigate this. It looks like a problem with PCIe 3.0 to PCIe 2.0 ..."
That was back on 30 March. I have not heard anything else, but I no longer care that much about it. Well, I sort of do, but a Mac Pro has been obtained for the porting effort, and I have moved on.
I would have liked to use the Mellanox in the Sonnet personally, but I will just port a 10Gig Intel driver for a card which is known to work, and use the Mellanox in my Dell server. Either way, I just want 10gig connectivity to the server from the mac mini, so it doesn't really matter which end has which adapter.
There is an other issue which precludes the use of the ConnectX adapter in any ThunderBolt enclosure, which I discovered relatively recently.
Thunderbolt device driver guidelines require MSI interrupt. Not MSIx. Not legacy INTx.
The connectX does not support MSI. Therefore, no Thunderbolt. Unless I wrote a polled-mode only driver. How much fun would that be?