Thank you Jorgen, didn't even think of the DFS aspect, that would make life much easier.
Ath3na,
While I won't have more than 150,000 clients, currently we have two business units, North America which I'm the SCCM admin over and have a primary site, and the UK who has their own primary site already up and running. They are wanting to throw us all under the same umbrella along with a new company acquisition down the line.
Things might get a tad messy just trying to force everyone under as secondary sites, and they no longer want to keep our primary sites separated, so a CAS with multiple primary sites is looking like the best option.