The thought had crossed my mind...I think the answer depends on how crazy you want to be. Most likely you could remove anywhere from 1/2 to 2/3 of the weight of the shell and still have an appreciably stiffer car. My gut instinct is 100lbs-150lbs of savings. There was an Italian guy on here several years ago who was talking about making a carbon bodied S1/47, but he did not ever post any claimed weights or price, and was looking for 10 interested buyers. I have seen pictures of a carbon bodied car from the Japanese outfit Body Shop Happy, but I am not sure if they just skinned a car, or truly did a 100% body drawn under vacuum.
If one was going to attempt something so bold, I would argue that it should be attempted as a full carbon monocoque so that you could truly take advantage of the material.
https://www.gt40s.com/threads/carbon-gt-mono.24086/ This guy is a total lunatic and he made a carbon GT40 from scratch. His day job is fabricating racing airplanes, so he had a good starting point.
Another guy who is a source of inspiration for my attempt at the panels is Mike Patey. He makes his own airplanes and does some very serious carbon work.
https://www.youtube.com/channel/UCSvdee86uThqIrloZjWwNVgIf you redesigned the underside of the car to maximally increase stiffness, and also incorporate the suspension pick ups in to the body I would guess you could easily save 200 lbs on the total weight of the car. But my god what a mammoth undertaking that would be.... I think doing a Mk14 out of the stuff would be a more fitting tribute to the worlds first composite monocoque car.