Sibs are definitely not more cold sensitive. Most varieties thrive in colder areas that get snow.
There are many differences in the morphology of beardless iris vs bearded. While under the same banner "iris" each species has different growing needs, different forms, and different zonal constraints.
I guess it would make sense to plant them in the spring if you are in an area with a shorter growing period and want them to be well established before it gets cold again. I am in an area with milder winters but hot humid summers. I plant them in the fall so they aren't stressed by the heat while first getting their roots established.
