The Spartina genus is a group of really tough grasses. Division and transplant is advised in the spring, and technically, it's still spring. I'd go fo it. Water it well all summer and fall, and provide some winter protection, and it should do fine.

I went for it. Transplanted two clumps, each a bit less than 4 square feet. They held together very well, didn't fall apart. The rhizome is amazing, when old it becomes tough as wood. The leaves look a bit messy now, even though I only broke a couple. I hope some new leaves will still appear to make the clumps look more established. We'll see, in any case I have quite a lot of spartina in my other beds so I will always be able to start over. Thank you :-)