Throughout most of America cake flour is 5% - 8% protein, pastry flour is 8% - 9%, and AP flour is 9% - 12%.
For my money, pastry and softer AP flours are best for pie crusts. Cake flour is too soft. Hard AP and bread flours are too hard. Pastry flour is also very good for biscuits; and to my mind it's the softness of the flour that distinguishes "Southern" biscuits from the generic.
I make my own pastry flour by mixing Swan's Down (skosh less than 7%) with King Arthur AP (un petit peu more than 11%), 50/50. Its close enough for government work. Another way to "convert" AP to pastry or cake flour is by subbing corn starch (0%) for some of the AP.
cake flour is processed with cholorine gas which "fractures" if you will, the surface of the starch allowing it to take on more moisture - making it High Ratio. ( great for a roux )
This will also help it to tend toward an upward movement while baking while pastry flour tends more toward spread and does not take on as much moisture.
Pastry flour instead of AP flour will lead to a more tender product.
Cake flour "and" pastry flour are both "soft" flours containing lower amounts of protiens and thus produce less gluten which results in a more tender product. Harder flours such as straight flour, pattent and clear flour are high protien/gluten producing flours suited for crusted breads. General purpose flour is formulated to be slightly weaker than bread flours (hard) so that it can be used in pastries as well. A professional baker however, prefers to use flours that are formulated for specific purposes because these give the best results.
The difference between "pastry" flour and "cake" flour is that pastry is slightly harder and is used for pie doughs and some cookies, biscuits and muffins. Using pastry flour instead of "all purpose" or "general purpose" flour will deffinately result in a different texture... a better one!