Don't forget the 328i economy numbers were adjusted when the EPA raised a flag. I think they're now artificially low.
It's never economical to take a low spec model and option it to match a higher spec model. Try configuring a 328i in an effor to match the 335i specs. You'll also find a similarly scant variation.
I'm sure the great majority of these 320s will have few boxes checked to keep the price down. When performance isn't a priority, people will buy them in great numbers.