I'm surprised you are only getting 20gbit/s. I did not expect PCIe to be be the limiting factor here. I've got a 100gbit cx4 card currently in a PCIe3 X4 slot (for reasons, don't judge) and it easily maxes that out. I would have expected the 25g cx4 cards to be at least able to get everything out of it. RDMA is required to achieve that in a useful way though.

Edit: forgot is isn't "true" PCIe but tunneled.

The limitation is Thunderbolt (32 Gbps theoretical limit for PCIe 3 tunneling).