If the parent tree survived Chicago winters, it's a safe bet the layer will too. The genus Pyracantha includes a large number of cultivars/varieties; many that do well in cold climates. I collected mine on the MS coast, but has done fine with little protection when outside temps dipped into the lower teens for a few nights at a time.
Time the separation so the new roots are well formed, but still have 6-8 weeks to establish themselves on their own before freezing temps. When you do separate, do nothing more than slip the undisturbed root mass into a slightly larger pot, and gently add bonsai soil. A variety that lives in your area should be fine overwintering in a hoop house.
Next year, let it grow unchecked, letting the roots escape out of the pot and into the ground if possible...they do grow hard, and it will be ready for anything you throw at it by 2015.
