XMR-Stak running 1 thread instead of 2 is about 15% slower than cast
Hmm, xmr-stak can use 2 threads, try this config for 6xGPU:
--opencl-platform=0 --opencl-devices=0,0,1,1,2,2,3,3,4,4,5,5 --opencl-launch=1152x8,1152x8,1152x8,1152x8,1152x8,1152x8,1152x8,1152x8,1152x8,1152x8,1152x8,1152x8 -o stratum+tcp://xmr-eu2.nanopool.org:14444 -u wallet_addr.$rigName/your@email.com -p x --donate-level=1 --print-time=5