This seems like a good spot for a brief discussion of Miller cycling, for those who may not know. Many of you N/I owners may have noticed your engine has no Exhaust Gas Recirculation (EGR) Valve. The purpose of EGR is to meter a bit of "dead" exhaust gas back into the intake stream. Although the gas is hot, because it has almost no oxygen, the effect is actually to lower combustion temperatures. This prevents the formation of nitrous oxides (NOx), which form only at very high temps. To avoid needing an EGR, most N/I engines instead use a technique called Miller cycling, where the ECM uses the cam phasers to close the exhaust valves late and open the intakes early, making the engine "re-inhale" a bit of the exhaust gas it just forced out. It has the same good effect on NOx, but without all the hideous plumbing that accompanies EGR systems.
The drawback to Miller cycling is when a catalytic converter breaks apart, or worse, partially melts and vaporizes. If any of the honeycomb debris gets sucked back through the exhaust valve into a cylinder, the cylinder is usually instant toast. Anything that comes out of a cat will be very abrasive, and certainly scored cylinder walls are the least you can expect. Most engines that are victims of cat inhalation are waste-cases that head straight to the junkyard.