The problem with separate encryption units, and the reason there has been a shift away from them into encryption-aiding cpu instructions, is that in modern software, (typically, requests and responses in web browsers), you usually only encrypt quite small chunks at a time. When you are never encrypting more than a few kb at a time, the cost of moving data to the dedicated encryption unit can easily wipe out most of the gains, especially when you often could otherwise process the data in the same pass you de/encrypt it.
Coherent interconnects are going to be there in future consoles. Just make the encryption/decryption unit listen to that fabric.