Ok so just installed new Radeon 18.8.1 drivers.

Was using 18.7.1 drivers before with SRBMiner 1.6.4 as I play games on this system as well and wanted to use latest drivers for that. 1.6.5 gives slower speed on that driver(which is obvious as it wasn't recommend to use with it).

So now with 18.8.1 SRBMiner 1.6.5 still gives me slow speed with cryptonight but with 1.6.4 I get +10H/s higher hashrate with RX580. No changes for Vega 56.