Why? Because its choked by a particulate filter. It has higher emissions, lower power, lower economy and lower reliability just to look cleaner out the tailpipe. Despite looking cleaner because it does not smoke, its actually dirtier than the federal model because it consumes more fuel and has higher HC and NOx emissions.

Removing it is as easy as swapping the intake/exhaust manifolds, oil supply/drain tubes, exhaust down pipe, cutting the air filter tube shorter and re-clocking the intake/turbine on the turbo to meet the ports.