It depends on the type of driving you do. If you're doing city driving and never get over 35 mph then aero mods won't help as much as driving 70 mph on the freeway. I think most people don't necessary spend most of there time on the highway so the biggest bank for the buck is driving style and gearing then aero. Some of the aero stuff should be standard on all cars though IMO. Belly pans, some form of grill blocks and wheel deflectors. Oh and something that all cars should have is some kind of mandatory FE instrumentations.
