I just started using HiveOS and I have a couple rigs of RX Vega 56.  I used PhoenixMiner at first and OC would take effect but not good hashrates.  I switched to TRM and OC wouldn't take effect at all and only about 38Mh/s.  admmemtweaks would work via shell but still only upto 43Mh/s.  Now hoping it was related to the version I downloaded.  Also didn't know RX Vegas didn't need the specific version for them and seems they can use the latest stable image.  So I will have to pull and flash the drive and try again.