"We still have a Boeing Bulletin that mandates the use of AI below -40 but the ops manual says otherwise."
Mostly correct, though as I recollect, the Boeing Bulletin (which of course supercedes the Ops Manual) only requires TAI to be on in icing conditions AND in the presence of convective activity WHEN thrust is at idle UNLESS the TAT is greater than 10 deg C (ie on descent in convective clouds and TAT less than 10 deg C).
Hope that helps.
