I've been quite optimistic about Zen2 myself. I don't see why AMD can't move to 7nm and improve their architecture at the same time. If Intel can get 5Ghz, I don't see why AMD can't if they put their mind to it.
I spent ages tweaking my 2700X after getting it, initially with all core OC's of up to 4.25Ghz, stable as a rock but a bit too warm for my liking. I'm on air using a Noctua though, not an AIO or loop. After weeks of playing around with the CPU and RAM timings I ended up switching back to XFR and just upping the BCLK. Now I get perfectly stable boosting across all cores at 4.5Ghz with lower temps than a 4.25Ghz all core OC. Benchmark results were better too, which surprised me. Think I've only ever seen the CPU hit 71C at full load.
Only thing that concerned me slightly were peak volts but they're not sustained and I've just come to terms with the fact that it could degrade the CPU over time. The 4.5Ghz single core boosts make up for it as far as I'm concerned and I've had no problems so far. Hence my thinking about the 3850X if indeed it does boost to 5.1Ghz. Wishful thinking maybe, but I can hope.
EDIT: Actually thinking about it, BFV peaked at 74C I believe.