Hard to say as companies that do use seam hesitate to publish any info. And there aren't that many public site that are built with seam (except http://in.relation.to/Bloggers may be).
When I switched from plain jsf/facelets to seam/facelets my performance increased due to usage of conversations (I didn't want to use session and had to use request scope more often than I wanted).
What you have to be concerned is memory leaks as those are harder to detect and it's actually dependent on your project just as much as on the production-readyness of seam version you're using.
From what I understand the Seam development team would gladly look at any memory leaking issues for as long as you provide them with the data :)
Besides if you want to start project that I would consider getting commercial support...