The way it's tuned isn't linked to the item level dropped in the instance. At all.

Yes your right about the part where coil gets 28% ilevels easier because of gear upgrades are you go along, but that doesn't change how difficult it can be when you first step into it.

They could make 6-9 in full 90 gear exactly the same diffiuclty as 1-5 in full 70 gear. But that doesn't mean it "It will have to have level 115 gear drop". it could drop level 95 gear or even NO GEAR AT ALL and still be equally difficult in 90 gear.