Problems with DUAL Mining

Hey,
when i solo mine ETH with following settings (WIN10, 6x 1070 Asus Strix): 

60% Power
+600 Memory
+0 Core

i get around 180Mh/s @ 670W

When i start ETh + SIA with same settings
i get 160 mh/s eth and ~1600 sia

is this eth drop normal? thought it would be less influence.

any improvements?
TY