I’ve had similar issues lately and as stated in Robert MacLean’s answer, AviD’s group policy changes don’t work if you’re not logging in as the users.
I found changing the LSA Lookup Cache size as described is MS KB946358 worked without rebooting or recycling any apppool or services.
I found this as an answer to this similar question: Wrong authentication after changing user’s logon name.
You might want to look into the following system calls such as the following ones:
LookupAccountName()
LookupAccountSid()
LsaOpenPolicy()
You could use them to write a C++/CLI (/Managed-C++) app to interrogate the LSA cache.