Not a 'traditional' ecomod, soundproofing!
Why? We all know that the biggest mod is adjusting the nut behind the wheel - with new cars the mods are few and far between. If the nut behind the wheel has an easier/ more comfortable time behind the wheel, they're likely to get better results.
A quieter cabin is more pleasant. A stripped out race car might be lighter, but it soon gets tiring, and a tired nut soon loses focus.
So I've added a bit more comfort, I sound proofed my doors. The floor already had really thick (almost 1") of foam on it. The difference is noticeable but not night and day.
I've done a few cars now. I've learned that even though today's doors are really tricky, using as few pieces is still the way to go. Aluminium tape seals the edges to prevent them pealing and creating sound paths.
I also added a little mass loaded vinyl to the door skins.
Any economy lost to weight is likely gained back by improved thermal insulation:
