The last point I've reached is quite stable heavy 1920-1930h/s, single Vega 64 (1478/920 1100/910). But still I don't understand what it depends on.
{ "mode" : "GPU", "worksize" : 8, "alpha" : 68, "beta" : 64, "gamma" : 4, "delta" : 4, "epsilon" : 4, "zeta":4, "index" : 1, "multi_hash":960 },
{ "mode" : "GPU", "worksize" : 8, "alpha" : 68, "beta" : 16, "gamma" : 4, "delta" : 4, "epsilon" : 4, "zeta":4, "index" : 1, "multi_hash":1008 }
About 80-100h/s appears when playing with alpha 64-68 on threads.
It can start with rising to 1950-1970 but then it falls to 1830-1840 (which is very good too). But it can also start with straight 1920 and work with stable 1920-1930h/s with no issues.
And that's with usage of FX4330(4-core, 8M, 4GHz) in that same config:
{ "cpu_architecture": "auto", "affine_to_cpu": 0, "use_cache": true, "multi_hash": 1 },
{ "cpu_architecture": "auto", "affine_to_cpu": 3, "use_cache": true, "multi_hash": 2 }
which does 120h/s.
I think it's all about some megabytes of memory usage.