The guy was being a jerk, but there is a bit of truth in some of it, IMO.
I think it depends on the inital drag coefficient vs the new value after modification, and whether or not its doing stop and go vs highway driving. I did a complete grill and front aero smoothing with removal of wipers, antenna, and a smoothed "race tape" (that I use on the race car good to 150 mph) front end with minimal air inlet, but when i tested it with my 22 mi loop test I found no difference. I have to say it felt like it was doing better (ie less pedal for more mph) at 75 mph, but down at 60 it didn't seem to have any effect on mpg. From 20 ft away you couldn't notice it at all because the tape is a good match for GM red.
After that failure I didn't do a belly pan or side skirts, as those would have needed expensive materials, which I felt would not get justified by reduced fuel consumption over the life of my car.
That said, another guy with a Saturn said he did aero mods on his and they worked, so maybe its just me, or maybe the grill wasn't the one that really affected drag, because he did a belly pan, too. Unfortunately he didn't post what mods had what effect, or even what mods he has, so who knows.
I do feel it would make a heck of a lot of sense for OEM's to design cars with much lower drag numbers to start with. We shouldn't have to mess with such things to get better efficiency. THEY HAVE wind tunnels, I don't.
bob, unfortunately I couldn't do any testing to confirm the usefulness of the bellypan or sideskirts. To further compound the problem the car is/was being 50% driven by my wife. So no data. My seat of the pants tells me that side skirts might be worth the time if materials were free or nearly free. However a full bellypan may not be worth it. A pan under the engine compartment is a good mod IMHO though. It allows control of the grill, cooling, etc. It also insulates the engine in cool weather.
02 Saturn SL
for pics click the link below