[Zope] CMF/Zope scalability

I have a client who is interested in using the CMF to build a very large system. I am wondering where I could find some proven metrics on its limitations. In particular, how many members could be handled using the standard Zope acl_users methods. I am not worried so much about load capacity, that can be tested using common techniques. But I am not certain about the ZODBs ability to store large numbers of objects and the Catalog to handle searches over these objects. Do you know of any sites/research where I may find some answers? Naturally I understand that this depends on many other factors, but can anyone provide me with actual experiences? Thanks Tom