I can't help with the carbon vs glass issue, but I can tell you that it is possible to keep the weight of an ICF TK1 down to 8.3 kg using 4mm Paulownia and one layer of 125 gsm (4oz) glass inside and out.

The Paulownia for the deck weighed just 1.5 kg and the hull 2.5 kg, so unless you already have acquired a lot of cheap balsa, I'm not sure that the little bit of weight saving would be worth using balsa instead of Paulownia.

Unless your carbon is lighter than 125 gsm, you won't actually save any weight over 125 gsm glass. It will still need roughly the same amount of resin to wet it out, won't it?