Thanks for the reply. The 5ms was just an uneducated brainstorm, as it seemed close on -10 and close at -20, but still not quite there. The Uverse box sadly does not have any such settings, but I will try to access the 'secret' menu.
After much trial and error, this is important for anyone playing with the lipsynch setting:
1. Change the lipsynch setting
2. Turn off the 105
3. Turn on the 105
4. Repeat 1-3
This seems to be the only way that the setting seem to actually take effect. Just changing it without the reboot will not provide the expected results.