65,000 is not close to 100,000. It's closer to 50,000. CAS's introduce many complications, latencies, and a few gotchas.
Unless you have a specific, real need, installing a CAS is not a "safe" move. If they do go over, that means the org has grown more than 50% (that's a lot of growth) and you can simply add a CAS later with no penalty.
Jason | http://blog.configmgrftw.com