The gain would be that an attacker having a foothold on the internal network (by having a physical access or hacking a device on it) would be able to sniff and modify outgoing emails.
I’m a bit sceptical about the performance claim on modern hardware.
That said it’s not a completely unreasonable tradeoff.
I have no experience about what you are trying to achieve, but rdma and related technologies (infiniband, qlogic, sr-iov, ROCE) is not it. These are network technologies that permit high bandwidth/low latency data transfer between hosts. Most of these bypass the IP stack entirely.
Infiniband is a network stack that enable RDMA, it’s only vendor is now NVIDIA which acquired mellanox. Qlogic was another vendor, but it got acquired by Intel that tried to market it as Omnipath, but it was spinned off to Cornelis network.
Sr-iov is a way to share an infiniband card to a virtual machine on the same host.
ROCE is an implementation of the rdma software stack over ethernet instead of infiniband.