I have found that CF, at least in the weight I have used, seems to use more resin than glass. If there is any way you can vacuum bag so that you can precisely control the amount of resin and assure a full wet-out I'm sure you would save as much weight as you would using a balsa core. You'd need a good system to suck down a bag that big quickly enough to make a difference. You might even need to vacuum pumps, and definitely a big reservoir for a fast initial suck-down, which is going to be the biggest problem.

m