Personally consider the metal treatment on the Glock one of the best. Would not consider an aftermarket slide for the Glock. Barrels, that another story! My barrels are stock but according to those in the know (not me) if your going to shoot lead a good aftermarket barrel should be considered. It seems in general those purchasing quality aftermarket barrels (KKM BS) have seen better accuracy in most instances but not all. Now all this is according to the lists and sites I have visited
Tim