can multiple methods access this iVar if I set the value to YES in one, can I check the value in the other
Of course you can, even if you set the value to NO
. It is an instance variable and thus shared between all methods of one instance.
Would this be better for performance
No, unless you access the property very, very often, like 2^20 times per frame. Have a look at this Big Nerd Ranch post about iVar vs property performance. Usually the performance gain is not worth the loss in clarity.