I have just received a recall for my Audi A4 Allroad to have the NOx fix. The letter clearly states that following the modification there will be "no negative impact on engine performance, maximum torque, fuel consumption, noise and CO2 emissions". This statement is at odds with your recent comment that owners, having had the modifications carried out, are finding "a decrease in drivability, with the usable torque coming in at 300rpm higher than before".