AMD has already confirmed that there won't be 3D V-Cache in the launch lineup. Besides, it would likely reduce the ST performance as it's measured here (in Cinebench), as the 5800X3D does worse in Cinebench than the 5800X. I would not expect a Zen 4 CPU with 3D V-Cache until sometime next year, to be honest.
In his interview with HotHardware, Hallock tried really hard to downplay that 15% ST performance figure, saying "we were conservative on that number in like 4 or 5 unique and important ways," though he did not elaborate on what those ways were. He also repeatedly emphasized that they were working with an engineering sample without a fully optimized frequency curve, and suggested that hitting 5.5 GHz was easy for Zen 4, leaving open the possibility of even higher clocks (whether through OC or higher-end stock configs is unclear)