Buried deep within the footnotes of AMD's announcement of Rome, it seems AMD is claiming an IPC improvement of about 29% when comparing Zen 2 to Zen 1. AMD did not disclose exactly how much more IPC Zen 2 had over Zen 1, but it seems 20% or more for at least some workloads can be expected.
https://www.notebookcheck.net/AMD-claims-Zen-2-has-29-higher-IPC-than-Zen-1-in-certain-workloads.359556.0.html