Honestly I would be surprsied if duty cycle dropped at all. It would only be if they would be able to run it leaner. To reduce the amount of fuel needed, you need to lean it out or stop flowing as much air. Now unless they backed the car down power-wise with the headers, I.E. kept dropping boost until they made the same numbers as without the headers. So flowing less total air, but making the same power. That would be the only way IDCs would drop. It would only be minimal IDC savings, and would not be my tuning strategy. Actually taking advantage of what the header has to offer power-wise, I would expect IDCs to stay similar or even slightly more.