Milltek themselves admit this does happen as the Lambda sensor detects too much heat and throws up the fault code. Doesn't happen all the time so you may be lucky
i believe it is the the other way round. The Less back pressure, due to the bigger bore, and free flow cat reduces the temp differential between the 2 lambda probes and throws a CEL light.
I am in the same boat, i want to access the increased torque that the DP will give, but at the risk of a CEL light.
I may be totally wrong though, but that was the way it was explained to me.................