I always here people say they can't fix new vehicles because of all the electronics. I agree, most of the time the vehicle tells you what is wrong. I remember fixing my wifes 2005 Chrysler 300C. It came up with a code that said it was an EGR problem. At the time I had the shop manual for that car on CD. It gave a long series of complicated tests to check the wiring and circuits to see if it was a electric problem or if it was just the EGR valve. The EGR valve was only $30 so it was easier and cheaper just to throw the part at the car and see if that fixed, which it did.
Really depends on the problem. We get people everyday that get their cars "scanned" at autozone, they sell them the "offending" part and more often then not it doesn't fix it. That's where I come in. You got lucky, as the EGR's were a pretty known issues with most of the Chryslers in the era. You start mixing computers and mechanical issues and you get some really weird acting engines. Its not that they are harder to fix, its just a different way to go about it. Now I need a 5k$ scanner that gives me the data I need. Sometimes you might get by, and get lucky, other times, no friggin' way. Don't dumb down the profession, its as easy as welding, in that it looks easy and you think you know how to do it but when you get into it you realize the pro's earn their money.
Regarding the original topic- volumetric efficiency is what these people are after. The advent of VVT and all of its variants is a good example of them trying to up the VE of engines, turbo/supercharging is just another big step in that. The EPA is really pushing regulation for emissions and mileage. Now they combine everything, VVT, displacement on demands systems, turbo/ supercharging.