Well Ryzen 1 ~matched Broadwell-E IPC wise, and 3800X is 15% increased IPC on that.
So if your 5820K was clocked at 4.5Ghz, the 3800X would still be 20+% faster clock for clock. Should be a decent upgrade, not even including the vasty better RAM speeds you'll be able to get. Some Ryzen x570 motherboards showing XMP speeds upto to 4400Mhz; and Ryzen performance increases as RAM speeds go up, and latency lowers.
In AMD's notes it simply states previous generation, I don't think they've confirmed if it's one or the other
[^2 Testing by AMD Performance Labs as of 5/23.2918 AMD “Zen2” CPU-based
system scored an estimated 15% higher than previous generation AMD “Zen” based
system using estimated SPECint®_rate_base2006 results. SPEC and SPECint are
registered trademarks of the Standard Performance Evaluation Corporation.
See www.spec.org. GD-141]
We do get the Cinebench comparison with a 15% gain over the 2700X. I'm not sure how reliable this is for indicating IPC, though the extra 100mhz probably means first gen.
Either way, all good