Sounds like your only running a single thread if your getting 1200h/s... I started off using XMR-Stak then tried Cast-XMR even tried Claymore 11.2/11.3 now I'm using SRBMiner Cryptonight AMD GPU Miner V1.4.1 https://bitcointalk.org/index.php?topic=3167363 running an 8 x Vega 56 rig and a 6 x Vega 56 rig. With the newer AMD 18.3.4 drivers. Stock Bios but I am running 85% pptables and OverdriveNTool for the overclock/undervolting. Check out the SRBMiner thread there are a bunch of us AMD RX Vega users in there that can help with configuring for the best hash rate and great stability...