Naveen from Microsoft (Management Platforms and Service Delivery (MPSD) organization) shared an excellent write up on his real time experience of migrating 200,000 clients to ConfigMgr 2012 !!!
He says after reviewing multiple options for client migration and installation (more info on client migration options can be found here – http://technet.microsoft.com/en-us/library/gg712283.aspx), we decided to continue using SUP based client installation. We also based GPO assignment on security groups due to some unique scenarios in our infrastructure.