2000 Volkswagen Passat Check engine / emissions workshop

Check engine light recently went on along with the emissions workshop prompt. Took it a mechanic who provided fault codes 16805 Warm Up Catalyst Efficiency Below Threshold Bank 1 and 16815 Warm Up Catalyst Efficiency Below Threshold Bank 2. The mechanic states both sides of the catalytic converter are warming up slowly (3 min vs. 1 min) and thus need to be replaced for $1800 parts plus $500 labor. Unfortunately, I am 1 year and 1,000 miles over the 8/80k emissions warranty. Just 8 months ago I had both rear oxygen sensors replaced for the same check engine/emissions workshop problem.

Researching this it appears there is a technical service bulletin 01 07 07 Jan 22, 2007 2010372 "Update Programming for Catalytic Converter System" The description states "On-bard diagnosis (OBD) for the catalytic converter system may be set too sensitive causing fault codes with out a defect in the catalyst system." It also appears there could be any number of problems with sensors, pipes, hoses, etc causing the faults. The mechanic claims this is not a computer problem and is standing firm that the catalytic converter needs to be replaced.

What should I do next? I am loathe to go to a VW dealer but sure would be happy if a simple computer update fixed things.