Am not sure, mod just replied that they are testing it on zf2. So basically 5.1.1 is a maybe or maybe not.will try to squeeze out more information as far as I can and I will get back to you here when I have a more clear answer.
Asus has been cautious as to how they reply in zentalk after the lollipop ruckus/incident for 1st Gen Zf. However another post of another moderator sheds some little light on android 6 development as posted today October 20,2015.(see attached pic)