Stupid question: 160mm vs 180mm likely refers to just the rotor, not the calipers? In other words, if the 180mm version was out of stocks, I could simply source an alternate 6-bolt 180mm rotor?
http://www.clarkscyclesystems.com/product-range/brake-systems/hydraulic-disc-brakes/m4-hydraulic-brake-3162.html?att_color=40&attr_diameter=229&attr_wheel_option=224