Since the portal will be aggregating your seam application and other portlets you have installed, I would write the Seam app as a standalone then use it as a portlet. As far as performance, I am unsure. We are just finishing up with the Seam bridge that will allow this. It is working now and we will hopefully have a preview release within the next few weeks.
Thanks for your work:-)
I want to know when the seam bridge will be released.