Depends... Standard leeks are very hardy. But there are 'summer leeks' which are not so much. I think if you had those you'd already be seeing a loss in quality. If you know the variety look them up. Mulching them up will help them keep quality and blanch them a bit too - and there should be loads of leave to do this with now.
Kale is similar. There are a lot of types of kale. The old standards types of Russian and Scots' kale are both extremely hardy. Other types can vary. They salad/young green varieties that have come out more recently are not nearly as hardy. The popular Tuscan kale is pretty hardy, but not as much as the Russian or Scots'.
Both Kale and Leeks have overwintered here in zone 4-5 conditions for me.
Brussels Sprouts. Well, I was going to ask your question about these myself. I could not find definitive info yesterday... So I covered them with a cloth mulch last night. Got down to 19F, but probably colder in my garden ~15F (the thermometer is near the house). Our temps swing hugely day to night, as today it's going to be in the 40's, maybe 50's. They are fine, but were covered. Regardless they are very hardy. Incidentally you can pull up the whole plants and keep them in a cold area for a few months according to http://www.harvesttotable.com/2009/01/how_to_grow_brussels_sprouts/
My Portugese cabbage - couve tronchuda also did well - much like a collard, it's an open-headed type. Seemed to do just as well as the Brussels Sprouts.