I know this probably isn't the 200 server cluster you were looking for, but I run a Seam site that gets 100k+ hits/day with very little load on the server.
I HAVE been having some out of memory errors after a couple weeks up lately (relatively new issue), but I"m hoping by upgrading the jvm to the latest 1.5 will fix that as _11 had a known memory issue.
Performance wise and stability wise I think it's ready for roll out. But I also know you're looking for an account from a large enterprise with a large cluster and huge user-base. I'm curious about that as well.
Try contacting Red Hat sales - they should be able to provide you with references for Seam.