Don't forget that the changes may adjust back pressure and tune. I have always used the rule that a significant adjustment to the exhaust should be linked with a rolling road ECU map to optimise them.
Interesting that you say that. For about 100 miles after fitting the sides pipes my S1 was feeling a little off colour. However, after some intentional stop/starts within those 100 miles, the ECU seemed to relearn the new relocated exhaust sensor inputs and settled down to normal. Car did however seem a bit more lively but I honestly don't know how subjective that is
