Michael,
We use the same FQDN for both internal and external access. Externally, the record points to a VIP that runs on a pair of TMG servers, which then load balance between the servers directly as opposed to passing through the LTMs. Internally, the record points to a VIP that runs on a pair of LTMs. Are you saying that if the record is myexchangeservices.mydomain.com that the URLs configured on the CAS servers for all web services need to be configured to point to https://myexchangeservices.mydomain.com/ (owa, ecp, Microsoft-Server-ActiveSync, oab) for both the internal and external URLs?
Disabling compression at the IIS level didn't resolve the issue.