I have both tubs and can tell you that the carbon version is light enough that with a dimpled bottom lipo Orion race spec 3800mAh you need a lot of weight to get to the legal 1500grams we have to abide by. You need over 200 grams which is the difference between this lipo and the 6 cell 4600mAh Nimh I used to run.
From what I remember when I built the car, the difference is small enough that you wouldn't feel it by hand which I estimate would be in the low tens of grams, perhaps less than 50grams.
I woud have to dismantle the car to tell you exactly though and I am way too lazy to do that.
Team Greasy Weasel
The best upgrade to any car is some driver skill.