Both choisya and berberis can be cut back hard. Do it now......plenty of time to regrow before the winter.
Choisya is simple but try to leave some green shoots. .....choisyas,have a habit of layering themselves so keep an eye open for this.
Which berberis? One of the deciduous purple ones? Or evergreen like Darwinnii which has orange flowers in spring? In either case cut as hard as you like. (I reduced berberis Darwinii by three quarters once.....decided it would die and I could plant a camelia there,instead )......and it responded brilliantly.
Give granular feed and water well. Mulch too if you can. Good luck