Hi,
I don't have an answer to your question, but I wonder why you think that disabling passthrough authentication would increase security? In fact it would decrease security. When using passthrough authentication there is no need to send username and password over the network again, since it uses an already existing security token.
If you really want to increase security then you should think about enforcing Kerberos authentication on the vCenter server (this would help regardless of whether you use passthrough or not). There is an old KB article (http://kb.vmware.com/kb/1006611) about how to do this in vCenter 2.5, but I'm not sure if this still applies to newer vCenter server versions.
- Andreas