I am dazed and confused! Why would you want to add to a sorted collection without sorting? It seems to me to be a canidate for the more general collection class. So again why have an addLast: method in the SortCollection class??
For example, the #copyFrom:to: method makes an exact copy of the elements between some start and end index. The same sortBlock is used, so the ordering is guaranteed to be identical. The greater the number of elements in the copy, the more significant is the penalty to re-sort the elements that are known already to be sorted.
Better to be able to copy the collection asserting that the order will be correct, without the performance penalty.