Hmm...no matter how green you make a car, someone will find a way to trash it. There must be some way to reuse carbon fiber...
The polo engine weighs about 220 lbs. If the transmission weighs (estimated)120 lbs, with the other small stuff that's about 400 lbs that you carry over from the stock polo. You really need to have a car that is about 1000 lbs to be competetive and that leaves only 600 lbs for everything else. I really think that a carbon fiber polo with an aerodynamic body just can't compete with the other cars. You would have to have a 2 cylinder alumimum or magnesium TDI engine, the lightweight aero body, and more, to be competitive.
I remember reading something about a carbon fiber prius concept and found some more info here:
http://www.bloomberg.com/apps/news?pid=20601109&sid=aFSrkKiAyKAI&refer=home
Using existing prius hardward in the CF body, the concept weighs 67% less (and costs 100 times more than steel at current prices), and gets 92mpg instead of 46mpg in the existing prius.
So if you made an identical projection for the carbon fiber polo with stock engine and transmission, you would get about 1200 lbs and 150mpg (rough numers). Pretty good but not even close to the other competitors. Maybe if you went to a lighter engine/transmission and went all out on weight reduction, you could get 1000 lbs and 200-250 mpg but you're no longer using off the shelf components and it's not a carbon fiber polo.