I started mining zcash today using EWBF and have been playing with the settings for hours.  I've found that both core and memory need to be boosted for best performance.  I'm using GTX 1070's and getting over 470 per card so your 1080's should be getting higher hash rates than mine.  I currently have my core at +170 and memory at +375.  I've confirmed that lowering either the core or memory will reduce rates, so from my experience, both are needed.  I also set intensity to 64 on all cards.