Actually, I believe low SN '03's were brittle, not soft. That's why, when they failed, they shattered.
I took a materials class as part of the requirements for my engineering degree, and it was really helpful in understanding the crystalline structure of metals and the different types of heat treatments that are used to refine them. Basically, solid metal has a crystalline structure. Cast metal has an unrefined grain structure, meaning that the different types of crystals within the grain are completely random and have very little, if any order. Which is why cast parts are, or were in the past, prone to being brittle and breaking. Forged parts start out as a hunk of metal, which was probably cast, but then the hunk is literally pounded into a certain shape by a press or hammer that exerts several tons worth of force. This pounding action refines and lines up the crystalline structure. The part is then machined to final dimensions, and heat treated.
Heat treating, however, can do wonders for a crystalline structure, or lack thereof. A fairly soft receiver, which is needed for machining, after being finish machine and going through the heat treat process, will be hard on the outside while still ductile at the core of the metal. This was the problem with the '03's, no ductility. It can also create or refine a crystalline structure where there was none before, which is what they do for cast parts.
FWIW, I don't like cast parts. However, I see that it is the way things are going in the future, especially with the prevalence of MIM, which is basically a fancy form of casting/sintering. I'll still go forged is I have the option or money.