As Jeff states there are really no Thunderbolt switches which currently limits the size of the cluster.

But would it be possible to utilize RoCE with these boxes rather than RDMA over Thunderbolt? And what would the expected performance be? As I understand RDMA should be 7-10 times faster than via TCP. But if I understand it correctly RoCE is RDMA over Converged Ethernet. So using ethernet frames and lower layer rather than TCP.

10G Thunderbolt adapters are fairly common. But you can find 40G and 80G Thunderbolt ethernet adapters from Atto. Probably not cheap - but would be fun to test! But ieven if the bandwidth is there we might get killed with latency.

Imagine this hardware with a PCIe slot. The Infiniband hardware is there - then we "just" need the driver.