Re: Epyc 7351P @ $700
Epyc has 128 PCIe lanes every model, Thread Ripper has 64 hence the different socket.
Epyc has models with a larger L3 cache then TR
Epyc can be dual socket, TR cannot.
There is, currently, no information regarding in memory encryption for TR but is unlikely since as you say it's an enterprise feature.
Even without those features Thread Ripper is sure starting to look like a real beast.