jesse, i don’t know where you got that you can substitute corn starch by doubling the amount with cake flour. i have a feeling it will apply mostly for cooking but definitely not on sponge cakes.
on a sponge, i would substitute by weight (equal weights), if you don’t have a scale, then i would substitute by volume.
i keep my cornstarch in the freeze, vacuum bagged, it may loose thickening power, but for sponge cakes i don’t think it is critical. otherwise cornstarch should not be kept for longer than 1 year. be sure to let the bag reach completely room temperature before opening it, otherwise you will have a rush of moisture going in.