Check out our scalability white paper near the bottom of this page:
http://www.broadleafcommerce.com/scalability.
You mentioned CMS. We have not done extensive load balance tests on our CMS domain specifically, but we showed pretty close to linear scalability in the linked white paper on a Broadleaf catalog with millions of products. We imagine that specific tests on CMS (structured content and page) would have very similar results.
By the way, as far as multi-store features go this is enterprise functionality but it is in active operation whenever you create a private demo site from
http://www.broadleafcommerce.com/demo. What we actually do behind the scenes is provision a brand new store on the same Broadleaf instance which copies test data over to specific site tables. We are currently are running ~1,800 unique private demos.
If you are interested in the multi-store functionality, shoot an email to info AT broadleafcommerce.com.