Is cornstarch and AP flour really a good substiture for cake flour?
It all revolves around gluten and gluten chains.

Cake flour is low protein, and bread flour is high protein, and everything else lies somewhere between. Individual brands have different levels of protein depending on their formulation. That protein, when combined with water, is what makes your stretchy gluten chains, and those are the difference between soft crumbly cake and a french baguette that could serve as a weapon in a pinch.

As with many chemical reactions in cooking, however, you can interfere with the "natural" reactions by means of some clever chemistry. Cornstarch works very simply...Corn just doesn't have any gluten, so you're just "watering down" the gluten content of your flour.

Acid retards gluten formation. Water is critical for gluten formation, unless you use a whole lot of it, in which case it weakens it (a wetter dough will make for a less gluteny final product). Mixing makes more gluten (helps the little proteins make friends with each other), so mix MORE for more gluten, and less for less. 

But fat is your friend. Fat waterproofs your flour, and keeps gluten from forming.

Long story short, pick a method you're comfortable with, and stick to it. If you're used to using AP flour, you're going to have some headscratching moments when the cake flour doesn't behave the way you expect it to. On the other hand, cake flour will be more forgiving if you overmix.