I don't see any reason to not use iga back for an outside set. For head, I think bersail cap would be better until you get your hands on ocelomeh/leonine/anwig.
It's a pretty lousy trade, any properly constructed set that's gotten to the point where Atheling > Iga is your best swap is going to see essentially the same damage either way. Atheling is inv+1 and doesn't require you to precisely estimate the mob's AGI. If you're going to build for crits, set your target DEX somewhere where it's not a super marginal increase over a full damage set even when you're at dDEX=50.
What to use instead and what about headslot?
Also, do most WS in Rancor Collar inside?
Atheling, Centaurus or Moonshade TP Bonus, yes.