On a larger cam such as the billet one in Levi's engine, it helps the engine move more air. Think of it this way, the intake valves are open longer allowing more air in the cylinder. This coupled along with the using same turbo you will see less boost because your using a larger quantity of air.
The cams do not affect idle. The only reason a gas engine lopes is from the poor vacuum signal to the carburetor cause by the overlap in a bigger cam. This is not an issue with a diesel. The injectors don't give two sh*ts what cam is in the engine. Therefore you have a smooth idle. Same reason fuel injected engines idle smooth even with decently large cams.
They also open the exhaust valve sooner thus allowing the cylinder to empty faster and more completely. This also enters higher velocity air into the exhaust with the positive effect of quicker spool.