Okay, despite insisting on a 003 part number the breaker couriered p/n 0065457632, a newer Siemens made unit but one of the Retrofit/Catalyst parts. It was too late to have a moan so I quickly swapped it over and set R16 back to "N".

After a blast on the M40 I can confirm that the hesitation is gone . Top marks to zakh! However I'm still convinced that it is the wrong part number as the EPC on this page shows. Will getting a 003 p/n further improve my performance? Having found none anywhere I'm sure that Mercedes did fit non-003 p/n to non-cat cars which is confusing. Any experts on this?

Also does anybody know the proper conduction paste to use when installing these units?