Cookies: Canadian All Purpose Flour or Cake/Pastry Flour?
talks a bit about Canadian Robin Hood AP flour.
If a cookie (or cake) recipe calls for some corn starch, it is probably because they think AP flour is too hard their purpose. But often recipes are not that refined. They just call for the most common ingredients, even if it isn't perfect for the job.
You may have to try both kinds of flour in some of your recipes. The ones using AP may be a bit tougher, the ones with pastry a bit more crumbly. Or you might not notice difference.
I found this online for you:
U.S. and Canada
All-purpose flour (nationwide) 11-12%
All-purpose flour (Southern U.S.) 7.5-9.5%
Bread flour 12-13%
Whole wheat flour 11-15%
Pastry flour 8-9%
Cake flour 7-8%
Vital wheat gluten 70-85%
which to me suggests that nationwide, in the US, all purpose flour has the same protein content as Canadian all purpose flour. The exception is in the Southern US. And I've never seen a recipe in, say, Good Housekeeping, that says "for this recipe, if you live outside of the Southern US states, please substitute all purpose flour with cake and pastry flour".
Now, I'm not too sure how much an oatmeal cookie recipe baked in Canada made with Canadian all purpose flour is significantly different in outcome from the same recipe baked in Boston with American all purpose flour. I suspect there isn't too much difference. And I've never really noticed a distinction between an "American" cookie recipe and a "Canadian" cookie recipe. Nor a "Southern US State" cookie recipe from a "US" cookie recipe in a recipe book. But that's just me and it might be because I haven't actively looked for this.
I think if you use pastry flour, depending on the cookie, it will be much softer and more cakelike. This may be good for some cookie recipes but not for others, where you might want a chewy or crunchy texture.
Personally, I'd just go with all purpose flour. I've been baking up here for years with it and never had an issue.