The XPS should be fastened directly to the wall. Unfortunately, the long outdated "wisdom" of leaving "breathing space" is still hanging all around the Internet.

Instead of pt plates, I would suggest strips of 1" min thickness XPS under the plates, wood or light steel. I don't like the idea of cutting chemical laden pt in a basement .

Here is a link that might be useful: Building Science Corp.: Basement Insulation

Thanks! We are going to use 2" XPS which should fill that "breathing space" void perfectly.

The dimensions of the basement allow us to use full 8ft pieces of 2x4/2x6 without any cutting. With this in mind do you still suggest XPS under the plates? What added protection would that give that the PT plates don't?

XPS is unaffected by moisture. I've seen pressure-treated fences and posts fall over with rot in less than 20 years. If you're using wood baseplates, at least put them on top of minimum 6 mil thick plastic sheeting to stop the continual upward moisture drive from the basement floor. You can buy one-foot wide rolls of the sheeting made just for the purpose.