To reduce the NOx you would need to run leaner. NOx is highest when lambda=1. As lambda is increased the NOx curve has a negative slope and therefore decreases. CO is lowest when lambda=1. As the air/fuel ratio becomes leaner then our CO levels go up slightly.
Some good points you mentioned Jim of how to reduce NOx. Some other ways being used in industrial engines is water injection into the cylinder during combustion and miller timing or closing the inlet valve early to name a couple
