I agree that the 6-6-6 rule is not more restrictive than the FAR's. As I am to understand though, this is one like the naming of an alternate, which can be as low as 400' and 1 mile, is issued by the FAA to a certificate holder if they can meet the requirements. Like the 6-6-6 rule, there must be at least 2 crew members and certain equiptment before they will issue that opspec.
So yes there are some Opspecs less restrictive than the stated FAR's, but they are issued by FAA on condition that certain requirements be met, which insures a higher level of safety.