I have several growing in the foam type of pots in the northern Virginia/DC area and they overwintered fine. Our low temps were in the single digits this year.
I'd wait until after they bloom to transplant them

Siberians can be grown in pots. I have grown them in pots and in the ground both with no problems here Minneapolis. For pots good drainage and plenty of water for Siberians. Normal division is recommended for August/September, but they are not really all that fussy.

Taken from The Society for Siberian Irises . . . . . . . .

COLD CLIMATES: Spring is the best time to plant or divide, with August as second choice. This gives the plant a chance to establish a good root system before winter rolls in. WARM CLIMATES: Avoid the hot weather periods; many prefer the cooler fall period.

The depth of your container is important. I didn't see that but assume 16 inches is a good minimum.

Here is a link that might be useful: http://www.socsib.org/ssicult.htm

Thanks Everyone for the responses. The depth of the containers might be a problem. They're probably around 12" deep. I'm considering burying them a bit or covering them with a heavy mulch prior to the winter. Again, thanks for the info.

COLD CLIMATES zone 4 in Minnesota qualifies. I have been growing Siberians for some decades here, mostly in the ground, but back when I was selling perennials, I wintered them in 6" pots pretty successfully. I also successfully divided them both in the spring and in late summer, early autumn. What I did was essentially bury the pots and then mulch them, which should work for zone 7 since it worked just fine for temperatures below -20 F here. It doesn't work so well here for the Japanese irises, since they are quite a bit less hardy. The two types are sometimes confused, which is why I mentioned them.