[GWP]

I have a 2011 550i manual shift equipped with a BMS Stage 2, Evolution Racewerks catless downpipes, and Schteigen exhaust. Currently has 56k miles. After installing downpipes/exhaust, I get check engine light regularly. Under hard acceleration (especially in 4th gear) the car will lag and jerk and eventually fall into drivetrain malfunction. I took it in to local European car specialists and they encourage me to replace spark plugs. After replacing plugs, the same failure happens. Now they are recommending a carbon cleaning which requires engine removal. Does anyone have any experience with this? Please help