Since you are in a hurry, I'll hop in here while we wait for the experienced cooks. I asked something similar once because I haven't really baked cakes. As you may recall, annie1992 makes tons of beautiful cakes in fancy and regular shapes. She says that she just uses regular ap flour. Of course, she also uses all of her vast experience. I suspect everything will turn out fine.

A poppy seed torte doesn't sound like the kind of flyaway feather cake that absolutely requires Swansdown. Cake flour has less protein than AP flour. AP is actually a mix between high protein (like bread flour) and lower protein (cake flour). If your AP is a standard bleached flour, like Gold Medal or Pillsbury, or similar house brand, it should really be fine on its own for most cakes or any baking that's slightly denser, like my concept of a "torte". A "heavier" unbleached flour like King Arthur AP is more likely to have a bigger effect on the outcome.

My assumption is that the purpose of the cornstarch substitution is to lessen the quantity of gluten forming protein, while maintaining the bulk of flour. I might use potato starch instead, or rice flour, because they have less flavor, but more people have cornstarch. You probably can't taste the difference in a flavored cake anyway. It sounds like a good idea for maintaining the texture you're accustomed to, but I've never tried it.

Just subtract 2 tablespoons from every cup of AP flour. Forget about the cornstarch. And as PIllog points out, the result will be closer if that flour is not King Arthur and is bleached.

It'll be fine, it doesn't sound like you are making something "feather light", like chiffon. I use King Arthur All Purpose for nearly all the cakes I bake, but I do have a box of cake flour for the very light textured ones.

I have done the "sub" with the cornstarch and it does seem to work pretty well, although it's still not exactly the same as cake flour. And it's the one time that I actually sift, then sift again, measuring carefully.

Annie